If you are working on the floor, I would spread out a large sheet first and tape to hard flooring wth blue painter's tape (or pin to carpeting with large straight pins). The sheet will catch any overspray.
Smooth out your backing, wrong side up, on the floor. Your batting should always be bigger than your top -- 2 to 3 inches bigger on each side for a lap quilt or smaller, 3 to 4 inches for a larger quilt. Batting should also be cut bigger than the quilt top, about the same size as the backing.
Lay the batting on top of the backing, fold back half of it, spray that half of the batting, then fold it back and smooth it out on top of the backing. Once you are satisfied with the positioning, fold back the unsprayed batting, spray that, fold it back and smooth it out over the rest of the backing.
Once you are satisfied with the backing/batting, smooth the quilt top over the batting. When done smoothing, fold back the top half, spray the batting, thn smooth the top half on top of the batting. Do the same with the bottom half of the quilt.
If it's a large quilt, at this point you may want to add safety pins along the edge about every 6 inches. Smaller quilts usually don't need this.
The spray allows re-positioning for at least half an hour, so you can lift and move things around until you are satisfied. After it dries, the quilt sandwich should be very stable for machine quilting.
Spray basting is used primarily for quilts that are to be machine quilted on a regular machine (not a longarm).