直接將車開回加州，在加州的DMV一次性的更換車主牌照和州車牌

You can call your insurance company (if you already had cars) and tell them to cover your this car when you drive back. If you are buying this car from a private party, ask him/her not to "inflate"** the purchasing price to be appeared on the back of tiltle or bill of sale (which ever is appropriate), as it's the base the MDV will levy the tax upon.

(** I couldn't use another term to describe this but hope you understand what I am saying).