You just need to press the foil onto a lens with the pattern on the outside, shiny side to lens. Fill mold level or a bit more if you want it convex on both sides.
No need to remove new lens from foil at all, you can use it like that, the foil mold IS the reflector. The foil will stay stuck until you take it off.
OR you can take the foil off and use the lens as any other normal clear lens. Just trim to size. I use small files.
IF you want to keep the foil on, file the lens from the foil side toward the resin side, that way the foil doesn't pull off.