Depending on the size piece you want to put on, you can "float" it on over existing film covering.

Cut the piece to the design you want. Clean the area it is to go with alcohol. Spray with BLUE WINDEX and put the piece in position. When happy with the position, squeege out the excess Windex (an old playing card is ideal) working from the center out and let dry for 24+ hours. Then seal the edges with an iron on low.
If you make a mistake, just respray with BLUE WINDEX while it is still wet.

The BLUE WINDEX reacts with and softens (activates) the adhesive on the film.

The "float" method allows you plenty of time to align the piece without the possibility of an "oops".