Hey guys,

(Please note this is my first build guide so constructive criticism and questions are welcome)

While not an avid player of this game, I have found this build to be most helpful.
If used correctly (and as always, depending on the teams), you can carry your team to victory.
I've been in matches where we had no inhibs. and the opposing team still had all their turrets, but once I completed this build I was killing enemy tanks in 3-5 hits.
Take it or leave it, I'm not claiming it to be the best, simply the one I've found works the best :)

As stated in nearly every other guide, armour penetration is beast for MF; enabling her to take out tanks while laning with a teammate, or to quickly dispatch the squishies in the middle.

I've gone with marks, and quints. in armour penetration, but I have yet to really decide which seals and glyphs are my favourite. I believe it is subject to change LOL

I've gone with seals/glyphs for mana regen, and then swapped runes for increased dodge chance (believe it or not, when paired with the ninja tabi this can help a lot).

I like going 26/2/1, I am only lv 29, so once I reach 30 I will put the last point in defense.
This build, when paired with armour penetration, really lets her tear into opponents early on.
Also the added armour can really save you when soloing, especially in the middle lane!
I go with one point in ghost, but this is subject to change based on which summoner spells I choose. If I decide against having ghost, I put the point in mana/health regen - its a little, but it helps ;)

Ok I've broken down my item buying process. Doran's Blade is very nice to have early game, it helps her squishiness out a little.

Next I save up for a pretty pricey item, it is completely doable especially if midding, however if you find yourself pitted against particularly skilled foes, you should buy mana and health potions accordingly.

It shouldn't take you too long to get the B.F. Sword - this will help IMMENSELY! This helps in your quest for the Bloodthirster ;)

After the Bloodthirster, go for a boot of your choice, this really helps her chase down enemy champs, later on sell them when you can afford a second infinity edge, or second phantom dancer.

Next, get The Phantom Dancer, it will help you keep up with the damage you take, when coupled with the Bloodthirster.

After attack speed, go for crit. chance and more damage - the Infinity Edge.
This can be swapped with a second Phantom Dancer, if you find it works better for you to have more attack speed earlier on.

Essentially the goal we want to reach is 2 - Bloodthirsters, 2 - Phantom Dancers, 2 - Infinity Edge's.

This build will have you crit. hitting almost every hit for over 1300 damage (if you have 40 stacks in both bloodthirsters). Also, if both bloodthirsters have max stacks, you will be lifestealing over 400hp per hit!

This can keep you alive when soloing against tanks and other DPS attackers.

I understand most people love double up, but I prefere Make It Rain. I try to max this one first, followed by Impure Shots, then double up. This is merely my way, you can swap this for a more comfortable build if you like ^_^
I have found that early on, a high damage Make It Rain can drop a support or ranged character's health by 1/4 - 1/3 (providing they don't dodge), which is extremely helpful considering it also destroys the surrounding minions. Also Impure Shots is vitally important when laning against someone who can heal.

I usually go with teleport and ghost. I know most advanced players don't see the need, but it's great for back-dooring latter in the game once the fighting becomes less "lane oriented".
HOWEVER
I have found having heal, and ignite, to be a most effective way of midding. Most high level players will be overly confident in their ability to kill you, and will usually go head to head against you, a stand-off if you will. This is where heal and ignite come in handy, after you make it rain, and impure shots them, activate ignite and wait till you are almost dead to activate heal. They will die if you played you're cards right ;)
Play around and see which ones are best for you, if you want, swap teleport for exhaust as it can really help ;)

Pros:
-high DPS (Damage Per Second)
-movement speed early game
-farming
-she does a sexy dance ;)

Cons:
-Squishy
-Slowwwwww if hit
-Low mana pool early on
-Is usually the first to go in a team fight
-Magic annihilates her

While not the most complex build, please do not be fooled by the simplicity of the item structure. This build can and WILL save your team if done correctly
Watch out for champions with high dodge capability, and "leaping" moves (*cough*...jax...*cough*) as they can dodge your high damage and close the distance between you quickly.
This build is extremely dependent on your ability to quickly hit your opponents, so make use of "Make it Rain" to slow them down while they run away.
Please note MF is not invulnerable, I have actually tanked with her and swapped one of the phantom dancers or infinity edge's with a warmogs armour, the lifesteal will keep you alive, and your champion kills will max warmog's quickly! (note - this is only used in rare cases lol)

I hope this guide helps someone, It works amazingly well for me ^_^
If you have any questions I'll be glad to answer them.
&& be gentle lol this is my first time.