Hello Mobafire, this is my Guide to playing Nasus on Toplane. Though I am not the most skilled player and my Elo is not very high either i play Nasus a lot, often successful. Nasus Top may not be one of the most popular choices and therefore is considered weak often by people only looking at what the pros play in the big offline events.
I am hoping to show you that Nasus Top is a strong pick, along with the right team.

Abbrevoations:
AP = Ability Power
AD = Attack Damage
HP = Hit Points
MR = Magic Resistance
CDR = Cooldown Reduction
Pen = General term for any kind of Penetration, may it be Armor or Magic Penetration
CC = Crowd Condrol (stun, slow, silence, etc.)

Future Plans:
- Lane Matchups
- Team Composition (Ch. 3 "Before picking Nasus Top)
- NO JUNGLE GUIDE(I jungle only if i have to and if so, not Nasus)

If you have any suggestions on what else you want to know about, tell me about it and i will do what i can.

Pros
- very strong lategame
- combination of tankyness and high damage ouput
- enormous bait potential against low skilled players
- strong waveclear and pushing power
- good sustain in lane due to innate lifesteal and low mana cost on main abilities
- easy farming beneath the turret

Cons
- heavily reliant on farm
- not very mobile -> vulnerable to kiting and ganks
- easy to poke
- very weak level 1
- no hard CC

Before you pick Nasus for top lane, you should make sure that your team has another form of initiate, as a Nasus walking towards your team can always be stopped with a bit of CC. Good Initation Champions are for example: Malphite, Amumu, Leona, Alistar.
It would as well be very beneficial if your jungler has any stuns, snares or knockups, such as Twisted Advance from Maokai, Bandage Toss from Amumu or Three Talon Strike from Xin Zhao since you have no such thing as of yourself and it will help you to kill your lane opponent or force cooldowns/summoner spells with your gank.
You can run Nasus in very AD heavy teams as well because Spirit Fire reduces the armor of enemies standing on top of it, allowing ADs in your team to deal more damage.
Nasus is a bit weak against strong lane bullies, Elise or Yorick for example. You should not pick Nasus against a lane bully unless you have played him a lot and know exactly what you are doing.(In that case you would not need a guide)
You should also think about how you do against the other champions the enemy team has picked. If they all have champions that don't rely on autihitting as much as possible, Nasus is not the best pick because Wither will not have as much effect as against autohit champions.

Runes
The runes focus on early defensive stats since Nasus is particularly vulnerable in early laning phase. You can as well get some HP regeneration on the seals or some more resistances depending on your lane opponent. If you are facing a full AD team, you might as well switch the MR for armor or some extra HP. This guide features a balanced setup that can be seen as a basic runepage for people that do not have a lot of runes or pages.

Masteries
The masteries focus on defensive stats as well, but scales better into lategame with HP per level, %HP, %MR/armor and %damage reduction. The Flash and Summoner spell CDR focus on being able to escape from ganks more often. Movement speed and vision range on wards do the same with movement speed allowing to chase people and counteracts Nasus' limited mobility. The neutral buff duration is for the case that you actually happen to get one, because Nasus benefits greatly from both buffs.

You should always have Flash or Ghost to escape from possible ganks or pull a maokai/lee sin/vi into your turret.

Exhaust is good for the defensive playstyle of Lane Nasus, also you want to kill everything with Siphoning Strike.

Ignite is not the best option, but it might help you to kill people with escape spells.

Teleport is a good choice if you plan to splitpush a lot because it provides a fast way to get back to your team in case of a fight.

General Skill Priorities:
R>Q>W>E

Your Q, Siphoning Strike is your main damage tool and since your passive lifesteal applies to the full damage of your Q, it will help you to sustain pretty good. As you stack up more bonus damage onto this spell by lasthitting anything besides buildings with Siphoning Strike, you pick it up at level 1 to start stacking as early as you can. Killing normal minions will grant 3 bonus damage, big minions, champions and big jungle monsters grant 6 damage. In order to maximize the bonus damage on Siphoning Strike, you should try to kill as many things as possible with it. Do not use it to deal damage to your lane opponent unless you want to fight, meaning that you should not do it unless you are in one of the following 4 situations:
1. Your Jungler is coming for a gank or is already on the lane
2. You got ganked and you cannot escape anyway, if so, you should try to get them low to reduce your opponents advantage
3. You get dived on under your turret, you might actually get kills there
4. You are really far ahead and know that you can safely force your counterpart into his turret or back to base

Your W, Wither, is a massive attack speed and movement speed slow that can easily be used to disengage if you want to avoid a fight. Unless they have good gapclosers or want to burn flash to get you, your enemies will not be able o follow you. Since you don`t really need your E in the lane you should max W second. In a teamfight, a maxed Wither will cause an ad carry to deal roughly half the damage as well as slowing them down to 5% movement speed(basically nothing), disabling their escape so you should use it against them or any other autohit based champion(i.e. kayle, irelia, jax)

Your E, Spirit Fire deals an initial burst if magic damage in a large area, reducing the armor of affected enemy units and deals more magic damage for another 5 seconds. On rank 3, it will clear the ranged minions if they take the full damage, rank 5 will clear an entire minion wave. You can use this to push a minion wave towards the enemy. If you are defending your turret this will help you to get the damage from the turret onto your opponents, while pushing turrets, you can use it to keep the enemy minions away from your own that have to tank the turret. However, the mana cost on this spell is quite high, so it is to be used with care.

Your Ultimate, Fury of the Sands gives you a good amount of bonus HP for 15 seconds and deals magic damage equal to a percentage of the maximum health of the enemies affected, as well as giving you bonus AD equal to about 6.4% of the damage it deals for the duration of your ulimate.
If you are dived on under your turret, this will enable you to get at least one kill out of the situation and possibly save your life.

Since you probably want to build a Sheen on Nasus, you should decide what you want to do with it. Both options are reasonable and I will not tell you which one to buy, you should make that decision again and again for every single game. To help you decide, i will provide you with a comparative list of the stats you gain from each item.

Trinity Force(3843 gold): - AP: 30 - AD: 30 - Attack Speed: 30% - Crit Chance: 10% - HP: 250 - Mana: 200 - Movement Speed: 8% - Armor: 0 - CDR: 0% - damage proc: 150% of AD, single target - slow proc: 25% chance for 30% movement speed slow, single target

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Iceborn Gauntlet(3250): - AP: 30 - AD: 0 - Attack Speed: 0% - Crit Chance: 0% - HP: 0 - Mana: 500 - Movement Speed: 0% - Armor: 70 - CDR: 10% - damage + slow proc: 125% of ad(area), the same area slows the movement speed of enemy units by 30% for the next to seconds

BUY AND PLACE WARDS WHENEVER YOU CAN!

Starting Items:

Good/Even Lane Matchup: You can pretty much start with whatever you want here, as long as you have some Health Potion in there, i prefer to start with Boots and 3 Health Potions.

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Bad Lane Matchup: For defensive starting items you have multiple options, against AD damage, Cloth Armor + 5 Health Potions are good. Against AP damage you should go for either Crystalline Flask + 3 Health Potions or Faerie Charm + Rejuvenation Bead + 3 Health Potions. If your lane opponent wants to burst you in the early levels of a strong early gank is to be expected, Elixir of Fortitude + 3 Health Potions can save your life.

First back:

No need for defensive stats(1270-1455 gold): If you don't need any defensive items, go for a Sheen, some Health Potions and 1 or 2 Sight Ward

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Against AD(790-1605 gold): If you feel that you need something against AD damage, you can go for a Chain Vest or a Glacial Shroud, along with some Health Potions and 1 or 2 Sight Ward

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Against AP(470-1825 gold): If you need MR, you can go for Null-Magic Mantle or Negatron Cloak or, if you somehow got a lot of money, you can add a Kindlegem to the Negatron Cloak and the mandatory Health Potions and 1 or 2 Sight Ward

Complete some Items:

In order to keep you inventory clean and leave enough room for Health Potions and Sight Wards, as well as new items, you should consider starting to complete some items. Which item you should complete first depends on the contents of your inventory after the first back.

You have Sheen: Go for Iceborn Gauntlet or Trinity Force, depending on preference and/or need

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You have Cain Vest: Go for Sunfire Aegis or Glacial Shroud, depending on preference and/or need

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You have Chain Vest: Go for Iceborn Gauntlet or Frozen Heart, depending on how you want to use Sheen later

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You have Null-Magic Mantle: Go for Mercury's Treads against heavy CC or Aegis of the Legion for the good mix of stats and the aura(upgrade it into Runic Bulwark when you need it or you don't have other priorities)

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You have Negatron Cloak: Go for either Abyssal Mask or Spirit Visage, depending on preference and/or need

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You have Negatron Cloak + Kindlegem: Spirit Visage

Completing your build:

After completing your first item, continue to build them as you need. There is no point in a specific building order to use in every single game and it would take too much time for me to type out every single Option. The previous sections of this chapter should help you prioritize what you need.

Enchant your boots!:

In most cases you should do quite well with Enchantment: Alacrity or Enchantment: Homeguard.
If you have brought Teleport and want to use it as much as possible, Enchantment: Distortion may be just the right thing for you.

Laning, especially very early laning is not that complicated with Nasus. Get as many last hits as you can without putting yourself in danger. Trading is no real option early because Siphoning Strike doesn't deal a great amount of damage early and has quite a high cooldown on early ranks.
Whenever you can, kill minions with Siphoning Strike to stack the bonus damage.

Remember to place Sight Wards to keep you safe from ganks. You should place Sight Wards as you can see in the picture below. Although it is best to have two Sight Wards, you cant always buy both of them. If you only have one Sight Ward, you should place it at the enemy jungle entrance.
You should also position according to your ward coverage, if you don't have wards you should not be pushing the enemy turret unless you are completely sure where the enemy jungler is and that their midlaner can't come top.

Warding the blue team's blue buff entrance will cover Baron, invade attempts towards it, roaming midlaners and of course junglers on their way to your lane.
Warding the top tribush will cover roaming midlaners, your lane enemy on his way to place wards and of course junglers on their way to your lane.

When you have your first defensive item, level 2 boots and Sheen, you should as well have a decent amount of stacks on Siphoning Strike, enabling you to kill a turret with just one minion wave that contains a siege minion, provided there is no enemy champion to clear the wave. this means, all you have to do is to wait for your lane opponent to go back and buy and you can push the turret and start splitpushing, leaving the lane only, when your team needs you for team fights or pushing other objectives.

In a Teamfight you should Wither the biggest autohitting threat, which is usually the AD Carry.
Spirit Fire should be placed beneath the target your ADs are focusing because it reduces armor, allowing them to be more effective
You then proceed to either protect your team as a meat shield or hit the enemy carries with your Siphoning Strike. Remember to only activate Fury of the Sands when you are standing in the enemy team, as it will only deal damage for a limited amount of time and you don't want to waste any of that.

I hope i was able to help you, I would like to have your feedback as well in order to improve this and future guides I will make.