This is a Twitch guide, have fun with the rat!

This is a Twitch build/guide.
The first thing you do is to get first blood at any lane, recall. Then go back buy the first shoes and a pot, now you get a kill in the mid, the first blood is 100%, or at least when I do it. The mid is around 70%, if he got flash ghost, AND if you did both exhaust+ignite at the top he's probably going to escape.

The runes are quite standard, Mark Armor Penetration, Seal Attack SPeed, Glyph Attack Speed and Quintessence Armor Penetration.

I go for 24/0/6 for Improved Ignite and Exhaust. The expunge stack a little with AP, but that makes difference, and the magic penetration also gives expunge a little boost. Then I take 3/3 Resistance and 3/3 Hardiness for a bit more survivabilty, and since you usually don't die you don't need to have reduced deathtime.

The items, is quite easy, I ALWAYS goes for this, it gives you a bit survivability(don't need much since you're ought to engage last in fights), a bit slow and MASSIVE damage.

I take Ambush for first blood, Expunge lv 2, and I take one point more in Ambush because 10 second stealth is just not enough imo(If they have a sight ward or anything, you're going to get stressed and stress always(sometimes lol) leads to mistake, and take slow in lv 4. Then max out expunge for the last hits and then ambush for stealth+nice attack speed. ANd of course ulti when possible :D

Exhaust+Ignite for ganks/early kills, nuffsaid.

Pros: High damage, long stealth.
Cons: Squishy usually first target(that's why you go last, and if you do you're safe.

Red buff is the only thing you need, leave the rest of the creeps to other.

JUst last hit and slow if you think it's needed, and your ulti in teamfights and it's GG:D.

Last hitting minions, ganking around, stuffs carrys SHOULD do, but with Twitch it's more ganking than farming.

Have fun with Twitch!