Early Game

Now early game is gonna be extremely boring, all you do is poke enemies with your Q crit (use your Q on low minions for the second shot to 100% crit). Get dark harvest if possible with your E from long distances.

Now the fun begins when you and your support have your ultimate. CC + Miss Fortune ULT = Completely Destroy enemy team.

So Mid Game, Huh.
Well you finally made it to the mid game, Here you'll start getting stronger and stronger. Considering the fact that you may or may not have fed and enemy. Here's what you'll have to do.

If you fed an enemy: Depending on how fed the enemy is, they're probably stronger than you. So start team fights and start getting fed, you need to become stronger, steal every kill and any kill you can because you're the ADC.

If the enemy is WAY too fed to be able to be defeated, either focus the fed enemy get him out of the team fight then easily win. or just ff at that point.

Normal Mid Game: So you probably got ganked a bit by the enemy jungler, and maybe a bit from your own jungler. So you're somewhat equal, Look at your CS (Creep-Score / Farm / Minions) Take that in consideration and compare yourself to the other ADC, if you're doing better you'll have an advantage, if not, try to get more CS, help out in teamfights and get your support to help you.

Mid game Fed
Wow okay, you got fed and it's already mid game. You're doing great and you have to keep it up. You'll probably dominate everyone at this point and all you have to do is get even more kills. More kills = more xp = more gold = more items = more damage = DOMINATION.
So lets finish up with the late game.

LATE GAME! WELCOME TO THE LATE GAME! Here is where you'll truly shine as a dark harvest miss fortune, that or you're weak and you can't do anything, but scratch that,anyways! Keep up the good work and congratulations on getting to the late game. Here's the final stage.

35+ Minutes At this point you're probably full build. The only way to win is teamfights or split pushing, as long as 4 enemies are on mid, take your support and go on bot, take 1 tower, retreat, and so on.
It's gonna be hard for you to do anything if there are fed assassins as they can take you out in an instant.
Play smart and you'll win.

Anyways! That was my Miss Fortune DARK HARVEST rune build guide. Have fun!