It's the university that makes the decision about whether you qualify as an "in-state" student, so all you have to do is convince them. You have to prove that you plan to make California your permanent home, remaining there after you finish school. You of course don't have to follow through on those plans, but you have to look like you're going to.
Register your car/driver's license in CA. Register to vote in CA. Pay CA taxes. Close out-of-state bank accounts and open ones in CA. Make your CA address your permanent address in all cases...have university documents, bills, etc. all sent there.
Basically break all ties with your home state. Then after a year you file a petition with the university, and they'll grant you in-state status.