Here's how I did it http://www.skoolie.net/gallery2/album13
. You can certainly do it differently. This has worked well, so far. I ended up priming the wood and putting down self-adhesive tiles. Oh, yeah, if you get exterior sheathing like I did, and if you are putting down some kind of vinyl tile floor, you will have to go around and fill in all the irregularities in the wood with wood putty...and then sand them smooth. It might be better to go with B/C sanded plywood, since I know I spent at least $20.00 on wood putty, and that spread over the cost of the 8 sheets of plywood that it took to cover my floor almost makes up the difference in the cost of the better grade of wood...plus the TIME AND ENERGY you save. If you put down something like a laminate floor, you can skip the wood putty step, but that flooring has its own, different issues.
I put down self-adhesive vinyl tile. It is working...okay... It has issues with expansion and contraction. I used it because it was cheap (36cents/foot), but after I got it down, the sunlight caused it to expand, and forced the edges of the tile together, which caused them to buckle and bubble up. Sheet vinyl would have been better. After I put the tile down I read the directions: "do not use in sun-rooms or on floors that exceed 85 degrees...". Of course, that means my hot water floor heating system is probably going to cause me problems. As long as I keep the sun off the tiles, they stay down, but if some sun hits them and warms them, they show some evidence of lifting. It's not TOO bad, if it only happens briefly, but if I leave a shade up and they really get warmed up, I have to go around and roll them down with a rolling pin. Sheet vinyl would not behave that way...I don't think. It would expand and contract, but not in the same way as individual vinyl tiles.