Guide is in progress. I write for fun. PM if you like.

I like to play Nasus. As Nasus, you can 2-shot carries with the cane. But that's not why I like playing Nasus. That's not what makes a Nasus player. What makes a Nasus player is that you like to stack. You're addicted to stacking. When you get so fed up with your lane opponent harassing you that you just all-in and kill them so you can go back to stacking, when you tp bot to turn a double death bot to a double kill just to get the gold to buy that CDR for stacking, when you defend mid inhib tower after your team was aced just so the game will go on longer so you can get more stacks - that's when you are a true Nasus player.

LeBlanc has nothing to do with assassinating people, making flashy plays, or putting out huge burst. LeBlanc is about being the squishiest squishy on your team, more so even than the adc because you have no lifesteal. LeBlanc is about laning against a Lux and knowing that if she lands a single slow, you're dead; a single bind, you're dead; and she can randomly ult every minute from across the lane and if you don't dodge it, you're dead. LeBlanc is about knowing that if you get cc'd once across the entire map, from a bush, from a support alistar or blitzcrank combo, from a point-and-click vi or malz or liss ult; you're dead. LeBlanc is about being about to clean-up a late game team fight and win the game but getting caught in a fiddle ult and dying from full health and losing it instead. To be a LeBlanc player, you need to be addicted to the constant thrill of always being at essentially 1hp. Of always being behind. Of 6-1-5 being a losing score.

Look at LeBlanc's portrait. The confidence, the assertively-held staff, the black piercing face. This is not LeBlanc. The image behind her, sprinting to-or-from an instant-death threat - this is LeBlanc.

This guide discusses Mid Squishy AP Assassin LeBlanc. I'm sure there are many, many ways to play LeBlanc, different lanes, AD, tankier - some of which have guides and some do not. But this guide considers only building as a Mid Squishy AP Assassin - that is to say, someone building to maximize burst magic damage output.

There are many stats to consider that might accomplish this goal.

** Putting Out Damage

* Leveling Up Skills
base damage of skills from experience and leveling up
consider, distortion: sigil of malice goes from 100 + .6 AP to 200 + .6 AP on both instances of damage at level 11 - an equivalent of 100 / .6 = 167 AP - more than a deathcap's 120 + 36, 167 * 21.75 =~ 3500 gold worth of AP, more than 8 kills worth of gold

* Damage Amplification
direct damage amplification, such as double-edged sword 1.5% executioner 5%, and havoc 3% masteries from the offense tree

* Ability Power
affects AP ratios on your abilities, and bonus to your auto-attacks from arcane blade offense mastery

* Magic Penetration
counter-acts magic resist, effective on both base and AP damage

* Cooldown Reduction
allows casting spells more often, like attack speed on an adc
i know i said a "burst" build; it's still a "burst" build it's just nice to be able to burst more times in a fight

** Being Able to Put Out Damage

* Vision
whether knowing where targets are to kill them, knowing where threats to being flanked are to not get caught, or simply having the literal vision to cast the targeted sigil of malice

* Mobility
from movement speed directly, from cdr on your dash, from teleport - all both to get to targets to kill and to avoid being killed

* Mana
even though your ult is free, mana will be a problem

* HP (including effective HP from resistances)
yes, i said you're essentially always at 1HP and if you are caught you are dead; however you do need to be able to take some poke, including the poke you will take while killing someone with your combo

Priority Masteries:

* Arcane Blade
both farming and pushing (or wave-clearing on the defense) is a constant objective throughout the game, and using spells to do so wastes mana and puts you vulnerable on cooldown
arcane blade functions as scaling ad for you throughout the game you're not getting anywhere else as minions get stronger letting you accomplish these objectives - bonus damage with auto-attacks against champions is just a bonus

* Damage Amplification
Expose Weakness - 1% damage amplification after you hit an enemy with a spell - i heard it works on yourself as well as allies
Executioner III - 5% damage amplification < 50% health, 2.5% average
Havoc - 3% damage amplification
Damage is (Base + AP) * penetration * amplification, and amplification can only come from these masteries and some random champion abilities like vlad ult, so grab these for sure

* Cooldown Reduction IV
cdr lets you put out more bursts and helps your survivability with more dashes (and even more cc with more chains); it also is the only stat that has amplifying returns (e.g. if you already have 200 AP 100AP is only 50% more, but if you already have 20% CDR then going to 40% is actually +25% CDR (.8 cooldowns to .6 cooldowns, .2 / .8 = 25%)

* Dangerous Game
hp and mana are problems for you, and getting kills is your job - that it gives you both on assists also is a bonus

* Devastating Strikes III
although not as rare as damage amplification, magic penetration is still rarer than Base + AP; you want to be useful against tanks and squishies start with 30MR even with no runes, aegis of the legion, spells or 6th game items anyway

Filler Masteries:

* Ability Power
Mental Force III
Arcane Mastery
Archmage II
more AP is better than nothing

* Sustain
Meditation III
Alchemist
Runic Affinity
HP and Mana are problems early game laning when poking and being poked; I took these masteries at random to try and help both. Since I'm not buying health items, spending constant 35 gold for 150 health heals is something I actually do a lot. Other choices for some sustain could be feast, block, recovery,culinary master and I haven't done the math for these ones in particular.

* Summoner's Insight III
Any summoner spells you take are very powerful and having them more often sounds great. You're also practically forced to take this if you go in the utility tree. A reasonable alternative would be getting your sustain from 9 in the defense tree and taking juggernaut instead. +3% health is also nice, but i'm going with the viewpoint that if you are going to die you're going to die anyway, and more-often summoners lets you scale your damage output more than 3% health will.

* Phasewalker
In a full endgame build you might reach 400 movemenet speed, .5% of which is 2 movement speed. Of course, movement speed matters more as a difference from your opponent than an absolute, and even being 1 movement speed ahead of your opponent could turn a particular trade from a death to a kill. But I'm not at the level and think the 1 second faster recall will help more either getting back to lane or escaping a flank.

Firstly, runes and worse rune pages are not free.

The only important rune choice I recommend is 9 glyphs of scaling cooldown reduction - 15% at level 18. You're only gonna get one of grail or morrellonomicon which each provide 20%, and 5% from masteries. Earlier I said cooldown helps both your damage and mobility and has amplifying returns, so I want to hit the cap and 9 glyphs lets me do it. If you want to rely on blue you might take 3 glyphs for 5% and rely on blue for the other 10%, but I don't want to rely on it. Flat CDR runes don't really matter because you won't have the mana to really make use of them anyway early game, aside from being half as effective. By taking CDR runes you give up MR, which will be a downside all game. But 10MR isn't that much and the final 15% CDR is that much. In lane you can get grail for 25MR to go with your 30MR base, and later game opposing mages will view you with 0 effective MR regardless, you'll just have to tank incidental magic damage or get a defensive MR item last if necessary.

Seals:
You should have seals of armor in order to jungle and they can defend against auto-attacks for any champion. If you have IP to spare I heard springing for one of the health options may be most efficient throughout the game.

Marks:
Flat magic penetration is rare - 15 from sorcerer's shoes and 15 from haunting guise (which you won't get) - and it applies after the 41% magic penetration from void staff and devestating strikes (which is what you want, .6 MR - 23 < .6 (MR - 23)) and you'll probably have a set to re-use on any ap champions.

Quints:
Movement speed quints are cheap and usable on any champion throughout the game. You could instead go for +15 AP for an early game boost.

More important than the summoner spells you choose is how you use them. Summoner spells have different uses in the laning phase and the teamfight phase.

Barrier, Heal:
Basically functions as extra HP during a fight. Heal for only slightly less hp can also heal allies in a teamfight or the jungler in a gank; but is vulnerable to greivous wounds from morellonomicon, ignite, or a katarina ult. If you're worried about getting out-bursted either in lane or in teamfights by say an annie or a fiora one of these is a reasonable choice. If you're looking for epic plays with the aoe heal is a good choice too.

Exhaust:
50% damage reduction lets it function like barrier versus a target, arguably scaling better later in the game, and comes with a slow for chasing or escaping to boot; the downside being requiring you to be in range and able to target the opponent - won't work if fiora already pressed R! the adc you want to assassinate can still kill you pretty quickly and this skill could put a stop to that

Ignite:
When you go in to deal damage, you're going to get hurt, so going in to kill can be a priority - in lane or in teamfights. Your sigil of malice is on a short cooldown but does very little damage alone, distortion and chains a longer cooldown but can proc the sigil and do more damage, and your ultimate mimic a still longer cooldown but a key skill in getting enough burst for a kill even on an already damaged target. Ignite can be viewed as a super-ultimate, with a 10x longer cooldown than mimic but the final piece of burst necessary to secure a key kill - again in lane or in teamfights. Helping to counter healing affects is a bonus.

Flash:
Can make big plays like escaping a key gank or catching a key target. But it comes with a long cooldown and doesn't work on everything. I personally never take it because most people do.
Cleanse:
The same idea as flash, although on different big plays and a shorter cd. DOES NOT WORK ON SUPPRESSION
Ghost:
Same song, third verse.

Teleport:
Firstly, teleport can save your *** when behind in laning phase. You can get a free heal, mana regen, and buy without losing your tower. Contrast with ignite which will be basically useless if you can't get them in its kill range. Secondly, teleport can get you a kill anyway in laning phase with pressure on top or bot lane, helping to reverse a snowball in one of those lanes to boot. Later in the game the map presence of teleport in getting to objectives from a recall or split-push is nothing to be scoffed at either. I personally take this spell.
Throughout the game there are constant demands on your attention, farming, not getting hit by skillshots, etc.; and staying aware of the map is desireable but just not realistic. However, try to prioritize looking at the other lanes at least some of the time when your tp is up to make use of it. Interrupted by cc but not by damage; still try to back off a bit before using it.

Smite:
Can be used on objectives, can farm siege minions when losing lane, can help kill super minions, can help clear heimer turrets or malz voidlings etc. I can't think of an upgraded smite that would be worth the opportunity cost of other items you could buy though.

Clarity:
An interesting choice. In the laning phase mana sustain is a problem especially when getting out-pushed e.g. heimer, malz. The periodic mana boosts from this spell could let you survive or even spam your spells enough to kill the pusher so you can afford to recall and not lose your turret. Also fun potential for epic plays with the aoe effect like heal.

Clarivoyance:
The shortest cooldown summoner at 60 seconds, with global range and a fairly large vision radius. Can use at start to see item builds. Can spam early on to keep tabs on their jungler. Can use when your enemy midlaner roams to keep tabs on them. Can use to get vision in a bush for yourself or for an ally across the map. I take this spell.

Although you only have 5 buttons to press (qwer and alt for mirror image control), the "function" of the skill you have in mind when pressing them varies so widely that really you have many more than 5 skills. For example, you may cast distortion to escape, to do damage, or simply to proc a sigil; likewise, you may cast chains to cc, or simply to proc a sigil.

Skill 1: threat sigil of malice
Your q is a low-cd spammable ability (and the only one you have). The mana cost isn't entirely negligible but it is bearable. It's a targeted ability with a slightly longer range than your auto-attack. Use this skill for a little damage but mainly as a threat. Throw it on your opponent; how do they react? Will they back off? Will you bait out a long cd ability from them that would have trapped you if you had comitted? Will they overextend into letting you unleash a combo? Think of it like a growl. You can use to intimidate, and you can use when you feel threatened say when backed up 1v2 trying to defend a tower.

Skill 2: poke sigil of malice
Your q's damage is doubled if you proc it by landing another ability within a few seconds. Of course your other abilities are skill shots, unless you want to blow your ult to proc your q but without procing your mimiced q, which isn't necessarily a terrible idea. Nonetheless, if you see a way to proc your q you can quickly do so and then get out to put some damage on your opponent. As a squishy, the only way for you to survive is to kill them or make them back off by threatening to by bringing them into burst range.

Skill 3: kill someone combo
Although as I said this isn't really what LeBlanc is about, at some point your jungler will gank or you will be trying to defend a tower or you need to peel someone off your adc or you need to kill their adc; and at this point you will be expected to put out the highest burst you can. This is accomplished by landing and proccing both your sigil of malice and your mimiced sigil of malice. The mimic procs the original so that's handled, but how will you proc the mimic? The first option is to land your distortion. This is relatively easy, but they can still dodge it. However, your distortion is your gap closer, and if your target isn't an idiot it will be difficult to get close enough without it. That leaves your chains. Your chains leveled last do a pitiful amount of damage, but them proccing your mimic is vital to your burst. This can be difficult so have some practice, but helpful is that you have a few seconds to proc the mark - so you can throw the chains instantly or wait a bit to get a better shot. Finally, if you wait a few seconds before mimicing your sigil will be off cd in time to proc the mimic - although this of course gives your target time to escape and lessens the burst.

Skill 4: distortion as escape
Distortion makes a gap, can jump walls (although not azir ult or veigar stun), and can blink back if they chase you. And mimiced it can do it twice. However, it has a cooldown, and mimic won't save you because once you use another skill you can't mimic it anymore until it's off cooldown again. Use distortion to catch someone or to farm, your escape is g-o-n-e. Finally, you can't blink back when cc'd, so don't think the escape makes you invincible.

Skill 5: distortion as aoe
Distortion is your only skill that does aoe damage. Before it's maxed it's pretty bad compared to sigil (although mimic is a separate damage and does good damage), and after it's maxed or when mimicd it's about 70% damage. So, you can use it to farm when absolutely necessary, and you can use it in a teamfight to output as much damage as possible, if that's more reasonable than assassinating a single target. However, when defending a turret against multiple enemies, however tempting it seems that the best way to defend it is to hit them all with as much aoe as possible, it's really better to try to kill one person unless you expect someone else will be able to finish them off if you aoe.