First, you have to remove most of the seats. There are laws about how many seats a vehicle can have and not count as a "commercial" vehicle even if it's not being used commercially. There's a lot of nuance and variation between different states, and in some cases specific cities. As far as I've narrowed it down, the punchline is that 10 seats is legal everywhere in the US, and 15 is legal most places and doesn't draw much attention in the 10-seat states if it's registered in a 15-seat state.
Fortunately I don't actually want my bus(es) to have a lot of dedicated seats. One thing I learned on the road trip to Burning Man last year is that with so much open space, passengers are much more likely to be comfortable on the sleeping surfaces than the sitting surfaces, given that the beds are cushy and the seats are not. They even spread out on the floor with cushions and sleeping bags. That might change if I had comfier seats, which could happen when I add a sofa. For now, simply ripping out every seat in the rear section and half the seats in the front sufficed. The remaining seats are the fold-up seats near the wheelchair securements, as well as the few seats that have bolts through the floor and require someone under the bus to remove. I got one bus that far on my own on Sunday night and Monday morning, and it was the first that I drove to the DMV in San Francisco.
Along the way, I stopped by my old bus to get the kitchen cabinet, with sink and stove, a water tank, and the toilet. Along with a bed, which we already had from the trip, those things comprise the general unofficial guidelines for a motorhome conversion in CA. The actual law just says "permanently altered ... and equipped for human habitation", which gets interpreted by various DMV inspectors in different ways. So far I'm 3 for 3 on big vehicles with a raised mattress, self contained toilet, foot pump sink, and propane stove. People get by with less sometimes, including as little as a bed and a camp stove, but that tends to be much riskier to try.
On the way to retrieve the second bus from its parking in Oakland I also picked up J and D from their hotel. J was already on the hook for a day of work, since our original negotiation had him still full-time driving through Monday. I extended my deal with D for drive time to include wrench time.
The first major wrinkle in the plan developed shortly after I got the second bus to the DMV parking lot, while I was still making logistical plans with J and D. While I was inside the DMV, J and D were going to be busy removing and stacking seats from the second bus. Interrupting that planning, a DMV employee came out and asked me to move my buses out of the parking lot. I said that I couldn't, in not so few words. Her manager came out shortly afterward and repeated it as a demand, to which I calmly and politely responded that I couldn't drive them on the street again until they were registered. I offered to move them to other parts of the lot, but she didn't like those ideas (those parts might still be needed for DMV operations later in the day, like motorcycle driving tests). In the end she said she would call California Highway Patrol (the lot is technically state property, being the DMV's) and I went back to planning.
When I had a free minute, I found someone in line just keeping a friend company. I talked to that person and gave them $10 to stick around in line to the end, with promise of $20 more at the end. They were happy to take it, and seemed incredulous at the deal. Overall, that $30 saved me about 2 hours in line, and is probably what made the difference in success or failure for the day. With that settled, I went back to getting the first bus ready. I cleaned it up a little and rearranged the living facilities.
While doing that, two CHP officers arrived. I greeted them outside the bus and they seemed friendly. Apparently someone had implied to CHP that I was somehow driving "both buses at the same time", which they repeated with incredulity. We talked about my options, and one of the officers went inside to talk to the DMV manager. Some fuss was raised about the second bus not being qualified for conversion yet, and the lead officer did not believe I could have it ready in the ~4 hours we had to get through the other process. In the end, he negotiated for me to move the buses to the positions I had originally offered to move to, with the caveat that I must get them out of the lot by closing time, and a promise from the DMV manager to sell me another one-trip permit to move the unconverted bus without plates (the same way I moved them from Iowa).
After a couple of hours I met my proxy near the head of the line and got some paperwork and instructions to get the bus inspected after filling it out. I had to ask a few questions; the form has changed in the last 8 months. I got it filled out pretty easily after that, then waited about an hour for the inspector. This is the same person who checks car VIN plates, and multiple cars came, waited, and gave up waiting while I was there. Eventually both inspectors on duty came out; apparently an opportunity for the senior inspector to do a motorhome conversion was a teaching opportunity for their junior counterpart. He was a little dubious that the kitchen cabinet wasn't bolted down, but happy about its general heftiness, and everything else looked good. He didn't ask me to demonstrate the toilet, which is sad because I love showing it to people.
Fun fact: the CA DMV website gives average wait times at each station, but those times start when you get a number. So, after 2 hours of my proxy standing in line, and another hour waiting for an inspector, I finally got a number and my official "wait" started, advertised to be about 30 minutes. As this was going on, J and D were finishing the seat removal (on which they did an amazing job) on the second bus. I asked them to swap the two buses positions and discreetly move the kitchen and toilet between them. I did the paperwork on the first bus, got its plates, and then went back to the "get paperwork" step for the second bus. Thankfully not the "get in line" step.
Inspection and paperwork for the second bus went smoothly. The DMV closes at 17:00, but handles everyone in line by 16:59, so we weren't even close to the last ones out at about 17:30. We drove the two new buses to the same block where I park my old bus, did a little bit of tricky maneuvering, and declared an end to the bus handling for the day. After dinner at a local pub J and D both went their separate ways, towards overnight plans and/or the airport.