I don't use after market masks Ian, and for camouflage, I just mark the pattern in pencil and then spray to the lines, but most mask sets are made from a similar material to Kabuti tape, the type that Tamiya masking tape is made from. Others are made using a vinyl type of material, and in both cases, they're normally pre-cut to shape (normally on a laser plotter) on a backing sheet. Peel off and use, basically.
Some of the other guys will probably chip in with more details I would think.