I use an old sheet on a table in a community room. I like to clamp the edges so they don't move, although I have done it without clamping. I always take blue painter's tape with me if I need to secure edges here and there.
When spraying, it helps a *lot* to spray from the edge towards the center to minimize overspray.
If you are using a solid floor (not carpeted), then I would use painter's tape to secure the sheet to the floor. Painter's tape might even work on carpet, although in that case pins should work too and might be better (as long as you don't miss a pin and leave it in the carpet!).
Instead of putting the backing down first, in this situation I would put the batting down first, smoothing it over the sheet. (A lot of people do it this way.) Put marking pins in the middle of each batting side before laying it down and smoothing it out. The batting is unlikely to shift on the sheet once you have smoothed it out. It will cling better than fabric.
Place marking pins in the center of each side of the backing and top too. This makes centering much easier.
Place the backing on top of the batting, matching centers so it is fairly evenly placed. Fold back half of the backing, spray the batting, then smooth the backing back over the batting. Do the same with the other side of the backing.
Flip the sandwich over, so batting is on top again. This time smooth the top over the batting, matching pins. Fold half of the top back, spray, then smooth that half back over the batting. etc.
I mention the tip about positioning first, then folding back half at a time to spray, because this makes it *much* easier to get everything positioned correctly. First time I did this I sprayed the entire backing and then tried to position all of the batting on top; *awful*!!!