I have my EMT through Los Angeles County, but the rules are basically the same statewide. (The new cards all say "State of California" instead of the county of issue). If you're current, you will need to complete 24 hours of CE units and then take a hands-on skills test.
You can get the CEs and hands-on test done by taking a 24-hour EMT refresher course every 2 years, or you can take the CE units on-line and then go in for a skills test. I typically do my units at:
http://ems-ce.com/
Down here, CARE Ambulance will do your skills test, and I think AMR will as well for a fee. Call your local ambulance company and ask for the training manager. If it's a smaller company, they might do it for free if you're a volunteer somewhere. I volunteer with San Bernardino Search & Rescue, and I can sometimes get my skills though the W-EMT trainer.
Other than that, it's just a matter of writing a check and filling out the forms/providing copies of stuff.
Good luck!
Arclight