The above link is a great how-to for building, very easy...

and checkout calguns.net for laws, mods, advice etc.

A few dealers even sell built ar lowers already CA friendly, then just pick up a cheap upper and you are in business...
check out http://www.lanworldinc.com/Lowers.htm built lower, find any unthreaded upper and you are in business, and can still use detachable mags.