There's only one part where I have to disagree with ph00tbag:
You need to have started playing by the very beginning. ZSS requires agressive play, and waiting for your opponent to come to you is a bad thing to do.
When you have the suitpieces playing defensively is a good idea. Plus, ZSS can be difficult to approach with, considering her lack of options.
However, pretty much everything else ph00t said is spot-on. Mainly, where the f*** is the side-b? I don't mean to be rude, but that's like not using Snake's F-Tilt or not using Shuttle-Loop on Metaknight. It's her absolute best way to space and rack up damage at the same time, (aside from d-smash).
Also, stop using bad moves: F-smash, D-air, sometimes grab, sometimes jab.
Your tilt game should set up your grab game. What I mean is, you should punish your opponent with tilts first, and then when they see they're getting punished by attacks and start to shield, then you grab. Not the other way around.
Also, your game should have 2 focal points. When your opponent is below 100%, get their damage above 100%, meaning lots of side-bs, tilts, d-smashes, and juggling. When your opponent is above 100%, try to land a d-smash or b-air and get the KO.
Umm, that's all I can think of, gl with ZSS, she's a pain in the *** to learn, but it's worth it.