An alternative way... Not that there is anything wrong with Shadow Dancer's way....
No tape method: I make two tubes one tube for the top and one tube for the bottom (roll them to look like snakes). Note the direction if your top isn't square rolling both the top and the bottom the same direction.
Now spread out your batting. Spray the batting and unroll the "BACK" tube pressing it to the batting as you go. When finished. Flip it over. So the batting is once again on the top. Spray the batting, and unroll the "Top" the same way as you did with the back. :)