Im not done with the wordy part of the guide, sorry. Its daunting and I struggle.

Glacial Augment is really nice on Miss Fortune. It can be applied from far away off of a Q bounce, and makes you very easy to gank for and helps your support land skill shots with hard CC.

I really love Blade of the Ruined King on Miss Fortune. It may only have half of the AD of other life steal items, but the on hit effect makes up for that. By the time you obtain it, most champions have at least 1k health, meaning you do 80 to 15 damage on hit and about 40 when they are at 50% which is what you miss out on compared to BT or DD. If someone has 4k health it would do about 320 on the first hit. The on-hit also procs off of Double Up so when you auto, Q, auto you really whittle the enemy down. The percent health damage is also very synergistic with Lethality because they can't just stack health to combat your armor pen. Attack speed is important on any ADC for sustained fights, shoving minion waves, and hitting turrets, so its good that Miss Fortune gets attack speed on an item that also gives AD. Blade really covers MF's weaknesses on having low base AD, pairs well with her W late game, and Q. The only down side is the on-hit doesn't go well with her passive and ultimate abilities or Q if it crits, but at least it has some AD on it.

Runes:
I prefer Comet over Aery for starting E because E slows so its more reliable to land. Comet does more base damage and has a greater bonus AD ratio. In the early game its cool down matches E fairly well being 20 seconds and 18 seconds on E.

Manaflow Band is the usual. It allows you to use E, which is a high mana cost ability, a lot more often than not.

Items:
Most of the items are usual and I don't feel like they really need elaborating on. So instead I'll talk about what I love and don't love.

I don't like Doran's Ring start because it changes a lot of adaptive damage to AP which is weaker on MF than AD.

I don't like GA on MF because you build health making the revive less effective in that regard. MF is an immobile ADC so when you revive you're more likely to die again right away. I find building health being more effective and defensive.

Strip it all off with The Black Cleaver! It gives very nice stats and is synergistic with your R and Lethality. If the enemy doesn't have much bonus armor and instead has just Ninja Tabi, TBC is going to do more for you than Last Whisper would because LW only works on bonus armor. Also I see a lot of builds and MF's buy both LW and TBC, but I highly suggest against it unless you really need grievous wounds, but even then I would still build TBC and replace Lethality with Mortal Reminder, but honestly I'd rather rely on someone else to buy Morello's or Mortal Reminder. Dominik's is absolute trash with TBC because TBC gives health, weakening the bonus damage from Lord Dominik's, and on top of that the armor ignore and armor shred have almost nonexistent synergy.

Elixir of Wrath is useful because it adds more damage and life steal and lets you heal off of your R. Its on average good in any situation, but excels when the enemy has Thornmail or Defensive Ball Curl, etc. Elixir of Iron is great for tenacity if you are the focus of CC or assassins because it gives a massive amount of health to survive bursts of damage, then you can life steal back up afterward. Elixir of Sorcery is low key secret OP, because the 15 mana regeneration helps you spam E and zone the enemy off of turrets and then deal 25 true damage per hit on the turret. It also scales with E well giving it 40 AP damage per use and 25 true damage per use. Its great for Lethality builds because you don't buy Essence Reaver.

Skill Sequence:
E max is nice because each rank increases the base damage by 35 which is almost twice as much damage as Q. Early game base damage is important because you don't have stats on items to scale abilities that use scaling damage more than base damage. E also gains less cool down and more slow effect without going up in mana cost and has further cast range than Q.

Q max second because its still MF's strongest basic ability when used perfectly and the cool down gets shorter when you rank it up. You do get 20 more damage every time you rank it up, but the mana cost goes up too.

W max last because attack speed is only really effective in a small skirmish, pushing minions, and pushing towers, and the movement speed doesn't help if you're being targeted.
The attack speed only goes up by 15% but Q is essentially a 100% attack speed increase between two auto attacks. It feels kind of weird in her kit and would probably be better if she instead had another AD damage ability in its place, but its unique interaction with her passive and the fact MF is still a marksman makes it useful.

Rank up R when you can, because it lowers the cool down a lot and adds a bit more damage.

RIP in 8.11

Heavy AD plus crit = one shot autos on squishies.

With enough attack speed, and when in a clustered team fight or when fighting on a minion wave, use Q not on your main target as to proc Love Tap twice on other enemies allowing you to proc Love Tap on your main target 3 times in succession. With crit it can be devastating.

So you lock in MF early, and to your horror your top, mid, jungle all lock in AD champions, and then your support picks Pyke. Your entire team is full AD.
You are 2-2 out of 5 in your promos and cant dodge.
Let your allies struggle stacking Black Cleavers and help them out with Press The Attack instead.
The best thing you can do is play it out and try to have an effective build for dealing magic damage, and this is a tricky way to do it.

This is a build for Middle Lane.

My favorite way to play support MF is to just run with AD.
It especially works well when your ADC does a lot of magic damage, like Kai'Sa, Kog'Maw, Corki, Varus, even Ezreal, and the rest of your team is heavy magic damage too.

This is really selfish and if you want to play as more of a secondary ADC instead playing support.

This is the best way in my opinion to play MF as support without forcing your lane opponents to build armor and inhibit your ADC. It comes online a lot sooner than when playing with Lich Bane.