California Only Procedure:...

http://www.dmv.org/ca-california/plate-surrender.php

Selling a Used Car in California
In California, license plates follow the vehicle. This means no plate surrendering to the DMV after selling your car. The plates leave with the new owner. After handing over the keys be sure to protect yourself by submitting a Notice of Transfer and Release of Liability. And for other steps to follow, check out our California Title Transfers page.

總歸是到DMV去交一張東西，這種手續自己想一下common sense就能猜出來。