The (now defunct) "Six Californias" idea was very interesting. I wondered who drew the lines, why they were drawn there, and especially disturbing: who named the resulting states. I liked the idea otherwise, and feel a better first step is to find commonality amongst a region within California that has a distinct difference to rally behind.
With 1-in-8 Americans calling California home (38 million people), no policy, law or representation can satisfy all. At best, a 50% vote majority leaves nearly 19 million unsatisfied with the result!
I chose the Sierra-Nevada mountains, because the daily concerns living in a rural community are different from those in big cities! By selecting 15-counties that have no major metropolis but contain significant association with rural living common to the Sierras, they would be an excellent first step in splitting the state to better represent the citizens of the resulting states.
Other regions will likely follow, with a main concern that all of them will want to be labeled "Northern California" with the exception of the Southern most counties.
I believe the people living in the respective counties should decide, and I think the state of Sierra (if citizens select that name) will start the ball rolling!