Laning phase is going to be very dependent on the matchup. However there are some basic things that you should always do as AP Twitch.

VISION CONTROL:

One of the most important things to do in laning phase is to try your best to control vision in the 3 bushes in your lane. A common thing people will do to counteract your all-in's is buying a Control Ward to expose your location when you look for an all-in. Make sure to always check bushes before trying any plays as you can easily be caught off guard by having your location revealed prematurely.

KEEPING TRACK OF COOLDOWNS:
Not only is it important to try and track your opponents cooldowns on their summoner spells, but it is equally as impoortant (if not more) to track the cooldowns of their abilities. Keeping track of your enemies cooldowns is also one of the most important things for AP Twitch as many matchups rely on cooldowns being down for you to get your full combo off. For example, tracking the cooldown of Mel's W (Spell that reflects back onto the initial caster) is super important for your all-ins. If she has her W up, your poison will be thrown back onto you and the slow also will be applied to you as well.

MANA MANAGEMENT:

Twitch's abilities cost a lot of mana, which means you can easily run out of mana quickly especially in the early game. While runes like presence of mind can help you manage your mana a bit better, it's not going to stop you from running OOM. Try and save your abilities for when you're looking to all-in, shove a wave, or escape from ganks using your poison and stealth. Many people will run oom constantly throwing W's on their opponents which is not a viable strategy as the early game true damage you get from your passive is not worth the mana you'll spend.

When to Trade:

Trading on AP Twitch is a rather nuanced thing, as the match up can make this an incredibly hard task. However, on of the best things to do when trading is decipher whether or not you need to save Q to escape, or if you should use it to engage. A great combo on AP Twitch is Q (use this to enter the fight and get in range so that your opponent is not only not expecting it, but will have a much harder time escaping) + W (using this will apply a slow to them that makes it incredibly hard for them to kite you or run away) + R (using your R can keep you at a safe distance from champs that deal heavy damage, for example: Draven, Miss Fortune, etc) + AAAA (this stands for 4 auto attacks, typically you'll only need a few auto attacks to apply your max stacks of passive, however continuing to auto does help you get the enemies lower and into execute range for your E+Poison damage) + E (Saving E as your final ability ensures that you get the maximum damage off and therefore making sure you will have a much higher chance of getting the kill).