Like cutecotton said, it goes on the outside on the back. One method that Ive found works best aside from tape is cooking oil. Yes I said cooking oil. Apply just a dab on the back of the tank, just enough to cover the entire surface. Place the backing on top of this and use a smooth flat edge object like a squeegee to remove the air bubbles. Tape can be used on the edges to prevent it from curling. In my experience, using this method creates a more crisper background and the colors really come out.
Ed