I lay the backing right side down and secure it tautly - not tight. You don't want to stretch it. (Ask me how I know. lol)
Then I lay the batting over it. I fold down just over half of the batting on top of itself, exposing the backing. I then spray the batting I turned over with the spray baste. Next, carefully smooth the batting back into place. Turn over the other side of the batting until you meet where you already sprayed (this will be stuck to the backing). Spray the batting and smooth it into place.
Now, lay the top on the batting/ backing. Fold down just over half of the top. Spray the exposed batting and carefully smooth your quilt top into place. So the same for the other side. At this point, I usually let it sit for awhile to "cure." I usually don't move it for 30 minutes or so. I don't know if it matters or not.
I don't generally pin when I spray baste.
Good luck!