Twitch works well as a midlane assassin for many reasons:
•He has stealth (with a reset)
•He has high burst and DOT
•Good movement
•Range (since other assassins are melee)

HOW TO:
Twitch has high damage early game which means you can be aggressive.
After lvl 3 you should engage either like this, Q, stealth over to enemy, W, AA, AA, AA, E.
OR: W, AA, AA, AA, AA, E, Q stealth away.
Finishing Kills will be hard sometimes, so try not to spook your enemy out of lane (unless you want to) instead poke only a little at a time and whittle them down until you can finish them off. Since you have Ignite and your passive the Damage Over Time is really high which is good for finishing your almost dead lane opponent too.
Farm as much as possible so you can get big items after first back. Since you have Savagery it should be easy to last hit.
LATE GAME:
Twitch is truly an ADC, afterall, so he will have very high sustained damage in team fights.
Since the guy is basically made of paper you will need to be on the backline in teamfights behind tanks. Also spam Q during teamfights for the Stealth ad you never know when you
will get the reset or need to leave. Chances are if your tanks are good its a free pentakill.

When playing this, I suggest you party up as its not super hyper effective.
Try to get 2 supports, such as Morgana botlane and Lulu Top
Thresh Jungle synergizes well (or any cc tank jungler, check out my Tank Ekko Build)

Thanks for reading this far I hope this guide helps you have fun and win!

Thanks -Max