Bind "D" to Ignite and "F" to Flash! :)

The reason for buying "Sword of Occult" is that you'll be ganking alot! And you will either get +5 AD or +10 AD for each kill..
The other items give you all the AS, AD, MS and the small portion of lifesteal that you need.

Expunge is your most important spell, and in most matches you'll get fb if you have a decent partner. LvL 1 kill: Hide in bush with your teammate. As soon as the target gets in melee range, pop Ignite and start auto attack. If you know how to void-walk you should be able to get 3-5 shots on the target. As the target tries to flee execute your expunge and you will most likely get your fb.
Next you choose Ambush. Now you can use the LvL 1 method without using the bushes and you got an increased AS. This ability is also useful if you're ganking, flanking or fleeing.
The next lvls 3,5,7 and 9 you'll use to increase the damage you do with expunge which will grant you many kills and fill up your stacks from the "Sword of Occult".
LvL 4 you'll choose Debilitating Poison. This ability can be used to both offensive and defensive strategies. For example this can be combined with your LvL 1 method for some extra shots on your target which also will increase your Expunge damage. It's defensive mechanism is if your fleeing a whipeout and need the extra second before your stealth is ready. It can also be used to save a fleeing ally since it's radius is quite large.
The reason for not choosing Twitch's ultimate right away, is because it is best used in clashes for it's penetrating attacks or to kill a fleeing enemy with the extra range. These situations don't occur very often before LvL 11, so you'll be enjoying your other abilities until Spray and Pray will be needed. (The ability has a bit of a con, which makes it shoot the penetrating shot in a straight line on the target. These shots are very easy to avoid if chasing a target, making it very defect early game.

Twitch is a very good carry, but his weakness is that he can't initiate. This is because: Twitch will always be the opposing teams primary target. Your best team setup would be 2-3 melee initiaters of which 1-2 are tanks, and 1 assisting ranged. With this setup you'd be positioned behind the opposing team. 2-4 secs after your teams initiation you can engage and you will get 3-5 kills easily as the focus is off you, and you have free nuke.