I'm still building my plan of attack, and gathering materials, so take this for what it's worth...
I have been considering this very topic, and I am not 100% sure I have it down right, but here goes my thought process...
#1. Obtain bus and of course place to work on it. This is probably the most important step!
#2. Gut interior. Stripping seats, flooring, walls and ceiling.
#3. Fix any and all mechanical issues, and any / all rust issues. I expect there to be rust in the floor on almost all used busses.
#4. Strip / remove any / all "school bus markings" stops signs, those funny crossing guard bar things etc...
#5. IF you are going to do a roof raise, or remove windows and skin over, do it now.
#6. Once bus is closed back up after roof raise, or window removal / skinning... Insulate walls and ceiling. Using foam board AND spray foam, or spray foam only. Let your budget decide.
#7. Add flooring stringers, foam board insualtion, and decking.
#8. Cover walls / ceiling with your choide of materials. I will likely be using beadboard.
#9. Rough in framing IF you are going to be doing walls. At the very least for the bed(s).
#10. Rough in bathroom / shower.
#11. Rough in water, and gas plumbing including water heater, fresh, gray, and black tanks. Tank rough in may happen earlier along the process now that I think about it...
#12. Rough in electrical. This includes any planned datacom wiring, antennae, and repeaters like a WiFi Ranger and Category 6 cabling. Also build in your camera system if one is to be used.
#13. Install or custom build cabinets.
#14. Build / install counter top.
#15. Finish bathroom. Make all connections and test.
#16. Finish kitchen. Add stove, fridge, sink, make all connections and test.
#17. Build out / finish bedroom(s).
#18. Build out living / dining and seating areas of the bus.
#19. Planning a custom entry door? Build and install it now.
#20. Prime and paint the bus.
#21. Add HVAC. I probably placed this too late in the sequence... Rethink this...
#22. We've already done the electrical right? Remove the hatches, Replace with Fan-Tastic fans and covers.
#23. Finish out any remaining itnerior items aside from the next step.
#24. Customize to suit your needs drivers compartment, co pilot seating, dashboard etc...