1) Convert your bus.
2) Insure it. I use GMAC, they didn't look at it. I insured mine over the phone. The toll-free number is on their website. I wouldn't even want to guess at the cost if this is your first car. Mine's about $200 a year for basic coverage, but I'm 51 with a clean driving record.
3) If you've bought the bus in CA then you already have plates, otherwise go to the DMV and get a transfer tag that will let you drive the bus to the DMV. I think these are good for a coupla days, the DMV can tell you ot you can find out the details on their website. http://www.dmv.ca.gov/
4) Drive your converted bus to the DMV and tell them that you want to register your bus as a motorhome, that you've converted it.
They'll do a walk through inspection and either like it enough to register it as a motorhome or they'll tell you what
needs to be done to make it comply. There are no special forms or anything. Plates for my bus are $58 a year. i forget
what the other costs were to register it - I think sales tax, but can't remember what else. Took about 30 minutes, beginning to end, but I went to a slow office (Barstow).
Other folks may have had different experiences, but this is pretty much how it went for me.