The easiest way is to lay out batting first, then lay the backing or top on the batting. Mark centers first to make centering the fabric on the batting easier.
The most important thing at this point is to fold back half of the fabric. Spray either the wrong side of the folded-back fabric or the exposed batting, and smooth the fabric out over that half. Then do the other half.
At this point you can flip the sandwich over, so the other side of the batting is exposed. Smooth the remaining backing or top over the batting, again using centering marks to help you get everything centered. Fold back half of the fabric, spray, smooth, and repeat with the other half just as above.