Not Updated For Current Season
I usually take middle lane with twitch. This purchasing order is very strict but it pays off in the end.
In the guide below I know that not too many people like sitting there reading long paragraphs so I'll make this short and sweet.
The Middle Lane
Try to always claim middle whenever there isn't a Karthus or a Kog'Maw. Whenever you harass, try to invis. then walk up to them hit them once or twice (depending on how fast that person reacts) then back off and expunge. Don't worry about killing the middle laner as much as ambushing the bottom and top lanes whenever possible. At around level 8 I usually stay MIA and the other middle laner does the same. You just need to make sure you get more kills than them.
Try to line yourself so your ultimate hits everyone in the fights and wait about 1.5-2 seconds to go into the fights or you'll be focused. Use a Zilean to fix this problem.
Pros/Cons of Twitch
-His ultimate can be dodged if the target is in motion
-Focused in team fights
-The best team fighter
Lastly, farming: Try to hit a minion then switch to another minion and get 1 stack of poison for each minion then expunge. You can also use your stealth to obtain a quick burst a attack speed
When hitting a turret, try to stealth before to start shooting it for the attack speed boost.(Optional: use your ultimate to gain some attack damage so you can hit the turret harder)
In case of accidental feeding:
Change your build to a
<Berserker's greaves-->Phantom Dancer-->Phantom Dancer-->Infinity Edge-->Blood thirster-->Black Cleaver>
Then you can absolutely rape the team fights and get every single kill.
P.S. if your team tells you to stop stealing the kills, tell them to shove it.