I like to clip my top to a table, lay the batt on top and smooth it out. I then fold back half, spray and smooth it back down and do the same to the second half. I then smooth the backing over the batt, fold back half, spray and smooth it down and do the other half. The reason I do the back last is so I can make sure there are no wrinkles.
I haven't used basting spray since I discovered the Elmer's washable glue technique but I am still trying to perfect that.