Bidule, cut your film to the correct length, then crease it lengthways. Lay the film into the gap and flex the control surface the opposite direction (remove the linkage for maximum deflection). Iron the film into the gap (I usually do the fixed portion side first and then the control surface).

You can seal both sides of the gap, but usually only one side is neccessary, ie., you can seal the bottom side and get most of the effect. If you do both sides, the second side is generally for cosmetic purpose (hides the gaps and hinges). Hope this helps.

A lot of guys here have the procedure illustrated, (I know Minnflyer did) and you can do a search for sealing hinge gaps.