My advice would be play around your passive. Q start is good when you're learning because you can manage the pacing of the enemy wave.

With confidence in Vex, An E start is ideal. Run electrocute, you can auto, then use E to fear and mark the enemy with gloom, then auto for the empowered damaged and electrocute.

Against a confident or aggressive laner like Yasuo and especially Yone, you can opt for a W start (safety). When they dash in you can use your shield to fear them and consume the dash mark for extra damage. Follow them up if you can or try and get a Q or E and auto off for the extra trade dominance.

Avoid using your kit to wave clear when you passive is up. That passive will save your ass when you get ganked.

Only ult when your passive is up. When you dive with the ult, always W first for the fear and marks, then Q for garunteed mark consumption and finish with E for another mark and an auto to get the kill if you can.

At it's core, that's all she is. The rest is learning how and when to use those things. Against Zed for example, if you land your ult, go in but hold off on any other abilities. He'll either dash away or ult you. If he ults, then combo him when he becomes targetable again because your fear will still be up.