Hi,
this is my idea on how to play Miss Fortune.
NOTE: Guide is not finished yet, I still need to add pics&stuff ;)


She was a champ I actually only bought because I liked the idea of farming with those bullets like hell. After a few attempts, I realized that an AP Fortune has no future, and I tried many different dps ways. Her double-up was adorable so I gave her as much AD as possible, but that did not really work out well because she kind of lacked both mana and other damaging abilities (her ult needs good positioning which i didn't have then).

Through all that time i nearly ignored her w-spell, impure shots. I'll come to those advantages later, but important is: this leads my way to an AS MF.

Runes

For marks, i use armor penetration, as they are the most viable extra-damage-source. Glyphs are for increased manaregen, as you may wish to do some double-ups without instantly running oom.
Personally, i am a huge fan regarding dodging. I don't know how many times it saved me to dodge one attack and therefore not having my passive buff removed and being able to get away unharmed. If you choose dodge-runes, I would hardly recommend nimbleness in your masteries.
If you don't like dodging melee attacks, I would go with flat HP quints and flat MR-seals for huge early game advantage.

Masteries

Usual dps-masteries without CDR, because your abilities are only supporting you, you don't rely on them as e.g. Shaco does.
If you did not take dodge-runes, I'd recommend taking a look in the Utility tree, therefore choosing Perserverence and Awareness.

Items

I usually start off with Dorans Blade (even after the nerf). It gives you survivability, increased damage and a little bit lifesteal, which is pretty nice for ranged champs as they don't have to get in danger to do autoattacks.

Boots are necessary for a squishy champ. Upgrading them is very important and best done exactly after buying plain shoes. If you don't encounter a team with huge CC (in which case you would pick Mercury's Treads of course) or an only-dps-team (hello Ninja Tabi! Works well with runes and also saves you from pretty much minion-damage), take the Berserker's Greaves. That 25% AS are pretty good, and cheap.

You might notice you deal nearly no damage. With some last-hitting and probably some ganks (important: your ult is NOT the key to gank. Please, if someone is pretty low just go down there and finish them off. You have got slow, with your double-up a burst and most important: you are a dps char. Again, you do not rely on your abilities!), you should be able to afford your first and only BF Sword pretty soon, which you want to be evolved into your mightiest item, The Black Cleaver. Now, you have two three of flat ArP (remember your runes and masteries), so do not get last Whisper. This is a waste of money.
The reason why i choose flat ArP is, that it works on everyone. With max stacks, you ignore 66 Armor. Especially lategame, this is about as much as squishies have too. So you nearly do true damage. ^^

The Phage is an extra-survivability/AD-item, you will need it. But before you complete it to your Frozen Mallet, you don't want to skip Madred's Bloodrazor.

Right now, you have spend nearly 6k Gold, therefore you have the midgame to collect for your Madred's, having it finished in time for everybody getting extra-HP. I dont think I have to explain why to choose this item. Every AS dps should get that, no question.

Finish your Frozen Mallet, as you absolutely need that survivability and you can chase very effective by now. Don't forget its use to flee! Just turn around, give him one double-up and he won't be able to get you.

It's about time now, that you get into teamfights 4v4 or 5v5, so we're gonna buy Stark's Fervor. Awesome Aura, and that lifesteal is Pretty helpful. Not to forget: it's another AS Item!

I don't usually get this far with my build, because it's not a cheap one. As your last item, i'd recommend Banshees Veil or Guardian Angel. Both grant you great survivability, as you might get focused by now.

Skill Sequence

As early game is all about proper harrassing, you should max your main harrassing tool while laning - double-up. It's easy to use mid, because your enemy never knows when you want to shoot. Sidelanes are even more interesting, because you can get both enemies with one shot. Therefore you can use ones laning partner to damage the other, towerhugging laning partner. You will get some ridiculous ranges with it.

Your next skill to max is impure shots. Your enthusiasm for an AS MF comes from this one. Just think about it: you hit your target and the next hit does extradamage. This is awesome in at least three different ways:
- Miss Fortune does mainly physical damage. While this can easily be evaded by buying armor, the impure shots do magical damage. Lets take a quick example: imagine you have got ~1.5 AS, i.s. is maxed. If you focus a tank and you don't die quickly, you can at least fire for e.g. 6 seconds, which is 9 shots. After four hits, you do 5 more with i.s. stacked to max on your target. Which means 280 magical damage (4*14*5) within 3 seconds. Disregarding your AD. So the longer you keep hitting one and the same target, the more painful your shots become (it just stacks up to 4, but this is fairly enough :) )
- Usually, an AS-built champ doesn't look as scary as a crit build. And also you might think, you deal no damage to your target. But exactly that is your advantage: you are penetrating them more and more, and usually they realise too late that you actually drained their life like Fiddle. The disadvantage is: hit&run doesn't do much damage. But for those harrassing needs, there is your double-up :)
- Activating the skill gives you extra (surprising) attackspeed and (most important) decreased healing of your enemy. This is just awesome. As you might encounter Vlad mid, you just pop out one shot just before he drains you, and he will probably miscalculate his lifereg, to your favor.

Make It Rain stays at level 1, as it does no significant damage without AP, but is in fact your necessary slow. Always place it in front of the fleeing or chasing enemy as it takes a short while to trigger. Use it to save allies, use it to push faster or anything else.

Bullet Time, your ultimate, is extremely powerful. It is all about positioning and timing, you gotta know where your enemies want to go. As you might imagine, it works best when everybody stays together. Try to get the center of the battle nearly at the end of your range. Normally people want to get out of it, so the wider your range is, the longer they stay in it. I don't get these opportunities often, so sometimes I just use my ulti to get sb low so I can towerdive him.

Summoner Spells

...are only about fleeing . Exhaust is an awesome 1v1 spell, chase spell and escaping mechanism. I use flash mainly to escape into the jungle, with the passive you are fast enough to get away.

2 things before you downvote:
First: yes, this build works. And usually I end up having a 1:1 physical:magical damage output, and viewing the graphs, i often am the one who dealt most damage. Though, you will be support dps.
Second: Yes, there are 8 items in this build. Your first three items are cheap, you should usually end up having at least Hextechs. When I am fully equipped and the game doesn't seem to end very soon, I sell Malady and SotD to get BC and GA, which are two wonderful items for lategame.

More to be added.