Hi all.
First time actually publishing a guide so feedback would be great.

Anyway lets get started.
I got asked to write this by several people on my friends list after they played with / against me. This is my "Preferred" build, I still tweak it here and there depending on what the other team are building.
Twitch is a ganker, pure and simple. He can be the best support in a team fight but will generally get the last hit, either from his attack speed (its maxed very early with this build) his passive, or his E. So be warned, you will be whined at for being a "OMGKSNOOB!!" but I only get this from Jax and Xin players. The best thing about him is his stealth, 50seconds maxed, thats enough to move around the map undetected to get ready for a gank, or to pick off those who survived a team battled and didnt have the common sense to B.

If you are new to Twitch, DONT GO MID! You will over extend due to over estimating how awesome he is. Until you know what yours and his capabilities are, go top or bottom. You will still get kills but you wont roam as much meaning instead of ending up in the high 20 / low 30 kills you will most likely get somewhere between 15 - 20 (If you play safe).
The way I do it is I stealth when I am about to get to the bush, and !Check! who I am up against. Dont do blindly in there and think you can kill them both, Twitch is a squishy! However, if the other team goes on the aggressive get behind them and lay into them. Remember to Ignite and you should get the First blood and if you are lucky a double kill.

Tip: If they get wards early it is worth investing in a Oracles just to wipe it out quick so you can still gank and not feed.

Pros

Super Carry
Awesome Dmg output
Super fun to play
When build is complete you can 3v1 ans still walk away with 50% hp

Cons

Super-duper Squishy
You will kill yourself if they get Thornmail until you get Bloodthirster and Vampiric Secpter
Hard to build if you dont get the kills you need
Your team will whine that you KS (Not true as you do the most Dmg, apart from Trynd)
VERY expensive build

Some guides I have read will tell you to go for Expunge first. I have tried this, doesnt work for me, so I wouldnt recommend it. Of course it may suit you so if it does go for it. However
stealth if twitch's best ganking move, means you sneak behind the other team, scout the other team, line up for multi-kills ect ect. which is why I go for it first.
Secondly Expunge, This move is your killing move. Add this to your Ultimate and its multi-kill central as you get armour pen and it travels through all obstacles in a straight line, therefore if you line it up right you can kill everything and anything.
Third move You ultimate. Already mentioned this I know but I thought I would go into a little bit more detail. Dont be afraid to spam it whenever it is off of Cooldown, its there to clean up lanes and clean up the score board. but the best thing about it is this. You can take out towers with it WITHOUT being hit by them.
Forth move and the least important in my opinion is your Slow. Dont get excited, its a very short slow, can stack up so they can barely move yes, can make you get that one last kills to score an ace yes, but its so reliant on the very small area it effects so I see no point in levelling it up until the very end. However there is one thing that I will make a special comment on. Banshee's Veil it will remove that effect, so if anyone builds that, use it. Also, against Nocturne, hit him once, Hit the slow, then pound him and hit E.

As I have already said, this is an expensive build. So for those who are struggerling to get the kills needed to complete here are some tips. Get 2x Zeal, dont complete to full Phantom Dancers until the end. Only get 1 Bloodthrister and leave the other as more damage or Vamp Scep.
The most effective way I have found if I am struggerling against a half decent PUG is to get Blood and Infinity Edge as close together as possible. That way I out damage anything they do.
If you are seriously struggerling then I suggest that you jungle until you get IE or BT.

Before I go. Thanks for reading, I will be updating whenever I get the chance, or new ideas hit me. I hope you will give me decent feedback so I can improve this guide so everyone can enjoy Twitch. I know that this build is expensive and makes Twitch squishy but trust me, if you play smart you will get awesome games and nearly always positive KDR's depending if the other team just decides to chomp hard on you.
Also when I get to level 30 (lol) I will tweak it for ranked.

Thanks again. Have fun and go get some!