This is my first guide for any champion so bear with me on this. Usually Miss Fortune is played like any other ranged carry but I found a nice unique build for her that maximizes her attack speed and damage out put through auto attacks.

For runes I go armor pen marks because her late game damage will benefit greatly from these even though for awhile you'll be doing mostly magic damage. Although you could switch them out for magic pen but I haven't tried it myself.

Mana regen seals obviously to keep your mana up while laning.

CD glyphs because you want you impure shots up as fast as possible to use it as much as possible in team fights. Although attack speed in either your seals or glyphs could be possible, The items you have will give you an attack speed of a little over 2.4 end game when using impure shots.

And health quints just for the extra added cushion to your life.

I like to max out impure shots first for this build just because its what it is based around. The passive of impure shots adds extra magic damage to each hit. Then the Malady and Wits end add even more. Add in the attack speed activate from impure shots, and you'll be shooting holes through anybody.

I sometimes may get double up a few levels early just for a back up damage ability but I rarely use it with this build. The motion to cast it interrupts her auto attacks too much.

Now you may notice this isn't the usual Miss Fortune build. I have chosen to make a build based around her impure shots by using malady and wit's end and then pouring in more attack speed with some items of survivability.

These are your CORE ITEMS for this build. The malady and wit's end.
These two items are what I use to focus on Miss Fortune's impure shots. The rest is really up to the player and their play style, although most of what I added also compliment these items in a way to maximize the damage output of the Core build while not losing survivability.

I add in some life steal mid game and late game for survivability but also more damage and attack speed with the Blood Thirster and Stark's Fervor. I know that the passive from impure shots, malady, and wit's end doesn't add to the life steal(that would be amazing) but I've survived some crazy situations with these items.

Now the end game tank item doesn't have to be guardian's. I just like it because of the passive. Although if they are AP heavy you might go with Banshee's or maybe FoN. Your choice really. She just needs the extra survivability late game for sure.

Summoner Spells are you choice. Mine are just my preference.

So to sum it all up. Wit's End, Malady, Impure Shots. To maximize the damage of these three is the goal here. If things go right you'll have so many number flying up the screen you'll go nuts( in a good way).

You just have to watch out for anybody with burst damage. Akali has been a pain in my side with this build for example. But any other attack speed carry such as yi or trynd usually have no chance against me.