Twitch is a range AD carry which envolves extreme high damage after being feeded.

Take flash for escaping because of the low ressistance involved by the squishyness of the ad carries,and heal for the same reason.

Ina teamfight or in a 2v2 on a lane try to keep the Expunge for the last hit,feeding yourself very easy because of the damage dealt by the venom sht stacks applied after one or two basic attacks.You may better want to ks a tank instead an other carry or mage,but this may be very useful,because of the tons of damage that will be dealt after the kills you'll make.After building the attack speed items you could sell the doran's blad for an Infinity Edge or another Bloodthirster increasing you attack damage and/or Lifesteal for the win.

If you work as a jungler with Twitch you can very easily make ganks by using Ambush in a combo wth Venom Cask slowing the movement of the enemies and ensuring the elemnt of surprise gaining more attack speed and an advantage over the other junglers.Also you may want to buy some items that will make your jungling easier,so buy some vison or sight wards to be putted at Elder Lizard,Dragon or Baron,or simply maximize ambush first and try to verify the brushes and the important jungeling zones by walking around them with your movement speed boost.If somebody tries to steal crreps of the jungle from you keep the Expunge and try using it for the last hit.

Never use your abilities like Expunge or Venom Cask or Spray and Pray for farming because you will need them if you want to go backdor in other lanes,cuz you may be the most wanted champion on the whole game for your squishyness and annoying stealthing.