Wow...there's a lot of answers to those few simple questions!
Really, as Sean says, it just depends. We all approach this so differently, with so many different desired end results and such a difference in our talents and backgrounds that it's very difficult to give a step-by-step guide.
I think I spent as much time developing my floorplan as anything because it affects and is affected by everything. You need places for the waste tanks, the fresh water tanks, the batteries, the appliances, the beds, the water pumps, the bathroom, the bedroom, the propane tanks if you use them, the furnace, or the fireplace, or the heater, the water heater, and lots more.
The layout is also driven by use...weekender, full timer, short trips, long trips, just one or two or a whole bunch of folks, hot weather, cold weather, rural mostly, or lots of traffic...and the list goes on...and on.
Visit every site you can for school bus conversions and check them out. Look at the photos on the site here. Think of the commercial RV's you might like.
From a build standpoint it's the typical from the bottom up and the walls in. Most of us deal with the floor first (depending on what you're doing with your ceiling you may tackle that next), then the things that will go behind walls, then the things that get attached to the floor that need openings to under the bus (waste and fresh water primarily along with maybe propane and wiring). Then maybe appliances. Somewhere in here some walls get put in; some sooner to provide a place to mount things and some later to allow working space. Ditto for built-in furniture.
Honestly, its like a giant jigsaw puzzle. You just keeping looking for the next piece that will fit and try to think far enough ahead that you have a way to access areas you need to. Looking at how others have approached it on their web sites helps an awfully lot. In that sense the pictures are worth way more than thousands of words!