Assassin MF? PTA? Arcane Comet?

Your rune pages depends on the type of game that you're going to play, on the matchups and on the items you're going to build. While MF is easy to pick up, the hard part is knowing when to go with the standard build, assassin or other options.

Secondary Page

Your secondary page depends on the main one, but they can be changed as you like. You can play PTA with Domination second, or Comet with Precision (Domination with Cheap Shot synergize better with your E ). Also Ispiration with Boots / Biscuits and Approach Velocity . This last rune gives you A LOT of kiting potential. However, you can find some information and options also in the notes of the rune pages.

Press The Attack

You can play this build in EVERY MATCHUP. It is the most versatile one: you can deal with tanks, have great lane pressure, poke when necessary, kite extremely well and have huge impact on the map. Overheal is a must-have, however, if you are more comfortable with Presence of Mind for your reasource managing, you can go that way. In this case, Bloodthirtser is mandatory, in fact you can't loose your W passive as long as you are shielded. If your item sets doesn't inculde BT, get your overheal AND Legend: Bloodline for the necessary lifesteal. If You have to deal with heavy tanks, go for the Cut Down .
This rune page can also be combined with Ispiration or Domination with Taste of Blood and Eyeball Collection .

Assassin MF

Against FULL-SQUISHY comps, Assassin Miss Fortune becomes a BEAST, expecially in the late-game. The rune page CAN NOT BE MODIFIED. Those are the OPTIMAL runes to get an early advantage and snowball it in the mid and late game. With this build, you're not going to buy boots , because it's just a slot that can be used for damage. Instead, buy Rapid Firecannon to have an attack speed and movement speed boost. Your first item is going to be Prowler's Claw or Duskblade of Draktharr . The active of the first item is not optimal for a ranged champion, but the passive is ABSOLUTELY BROKEN. You Just keep gaining lethality, which is more effective the more you have. However, you can use the active to fight another ranged champion if you know you're going to win, leaving them no chanche to excape. You second item is going to be a Last Whisper, like Serylda's Grudge or Lord Dominik's Regards. Flat armor penetration is applied before lethality, so this stat is going to be even more effective. From now on, you complete your build with RFC and other situational items.
You have to be VERY CAREFUL: not having boots is hard, you have to be very precise with your positioning, because if you loose your W passive, you're most probably dead.

Arcane Comet

With the Arcane Comet Build, you're going to max you E first, starting with Tear of the Goddess and rushing Manamune. Then, complete your build as you like. Eclipse is very strong, but Galeforce + Comet is a godlike build. Anyway, this build likes to poke a lot, expecially in the lane phase. You're going to harrass your opponents and try do deny them farm. The slowing effect of of your E makes the comet hit almost 100% of the time. This is really good, because the comet cooldown is reduced when it hits.

Your lane phase with mf depends A LOT on your build but mostly on your support. You have to understand different matchups.
Press the attack is standard, you can play it almost every game and adapt it to every enemy team comp, changing the itams.
If you're against squishy enemies, you can go assassin and have lots of fun, gaining staks for the [dark harvest] even in the first minutes. If your support likes to poke, you can poke too with the arcane comet build.

General Tips

FARM FARM FARM. Everything you do, everywhere you go, don't forget to farm. If you want to roam, get a dragon or recall, PUSH YOUR WAVE. In this way, you opponent either misses his farm to follow you and help his team, or he'll arrive late and be useless. This is a win-win situation. Also, fighting in an enemy wave is going to lead you to death. Minions deal damage, and you take their aggro hitting an enemy champion.
You can also use your R Bullet Time to push a wave the fastest way possible. Use this in the early game, if you want to recall or if your enemy is missing and you want to make them loose farm and even get a plate in the optimal case.
Talking about your ultimate, USE IT AGGRESSIVELY. If you don't kill the enemy, he'll be very low and he'll have to recall. IMPORTANT: 'aggressively' doesn NOT mean 'randomly'. POSITIONING IS KEY, and you can change the result of a team fight with a well-placed ult.

Press The Attack / Standard Game

This rune page is the most versatile: you can choose your gamestyle and change it throughout the game, adapting it to different situations. During the lane phase, you have to farm like every adc, but also try to deny it to the enemy, being Miss Fortune a really strong lane bully. You want to be passive and try to catch the enemies out of position, so that you can deal damage or even go all-in, triggering your PTA and kiting the target to death.

Assassin Build / hard-engage supports

Sometimes, you want to be super-aggressive and often go all-in (Assassin build / supports that can engage like Pyke , Morgana or Alistar ). By doing so, you gain early Dark Harvest stacks. On your first recall, you wanto to buy a Serrated Dirk, gaining attack damage and 10 lethality. However, despite eing aggressive, you don't have to force fights or put yourself in a risky situation. Assassin MF is an end-game monster, and with time you'll win no matter what.

Comet Build / Poking supports

With the Arcane Comet build, sometimes even with Press the Attack, or poking supports, you want to farm passively and deal small amounts of damage when possible, trying to deny farm to the enemy ADC. You can use your E , which synergizes lovely with the comet, thnaks to the slowing effect, or you can use your Q (expecially the second part, the bounce) when enemies come near the wave. To maximize the damage, use your Q on a low-health minion, so that the second hit critically strike. If you are using Press the Attack, when the enemy is low enough you can easily trigger your rune by th AA-Q-AA combo and then cast your E for an easy kite.