In a nutshell, the way this build ends up working is by using a synergy between Wit's End and Impure Shots to create some cheap damage and hopefully score you some early kills.

Runes aren't a real big part of this build, but I'm recommending:

Greater Quintessence of Movement Speed for an increased movement speed advantage.

Greater Mark of Desolation for some Armor Pen.

Greater Seal of Evasion for a little survivability.

Greater Glyph of Scaling Mana Regeneration to help with mana.

Exhaust and Flash.

You're going to be very squishy and both these can help you survive long enough to hopefully get a kill or two.

For masteries I like to run an 11 / 0 / 19 setup. In offense I want to go down far enough to get Archaic Knowledge , Impure Shots, Make it Rain and Bullet Time are all magic damage. In Utility I want to go down far enough to get Quickness . The extra 6 points are up to you to decide what's best. For me Intelligence is right there for some more cooldown reduction and Sunder makes sense for a little more armor pen.

At it's core, this build is all about rushing a Wit's End. The faster the better, because it's on hit effect is most effective early on and gets weaker the longer the game lasts.

Doran's Blade or Doran's Shield are both fine options, I prefer to start out with a doran's item to help out in a solo lane.

Boots next will really give you an advantage when combined with your passive Strut and Greater Quintessence of Movement Speed + Quickness .

Wit's End is up next, it's not too expensive and is a great item in the early game on Miss Fortune, because you can combine it with Impure Shots for some really cheap damage.

Sword of the Occult is a good way to go next IF you are able to use your Wit's End and speed advantage to net some kills and assists, so don't just throw your gold away if the opposing team is able to escape your gank attempts.

Madred's Bloodrazor since you're already stacking on hit effects. The Bloodrazor will help you bring down those tanky hero's, while your Wit's End destroys their mana pool.

Phantom Dancer is about the best item I'm going to recommend for survivability, as you continue to keep your movement speed advantage.

Infinity Edge is nice way to finish out the build, but to be honest if the game has gone long enough to get this far, you're probably better off selling Wit's End and adding some items to help with your squishyness. ;)

It all boils down to stacking movement speed, attack speed, and on hit effects. It works great in the early part of the game, and gets weaker as the game goes longer. If you can get some kills early on you'll set your team up for a win, but tower diving on Miss Fortune is generally not a good idea because she is very fragile.

Good luck and happy hunting!