1.) The typical harass combo for Zed is WEQ to proc the slow first to make the Q easier to land.
2.) Before you go all-in at Lv.6 + it is best to WEQ and wait a few seconds for your energy to restore so that you dont run OOE (out of energy) while trying to pull off your combo and realize you can't cast your Q so you miss out on a kill.
3.) When being ganked by the enemy jungler and your W is down, remember if you cast your E on enemy champions it reduces the cool down, this can be very crucial in determining whether you survive a gank or not.

4.) Early game you can use your passive to your advantage by WEQ'ing to harass and if your opponent is low enough you can flash auto ignite for the kill, most people do not expect the burst damage and are caught off guard.

5.) Learning not to rely on your ultimate for kills is one of the most important factors about playing Zed, you should try to use your ult to dodge skill shots/cc/damage. Being able to take out a target using only your Q W & E make you much for valuable in a team fight rather than simply relying on split push.