The decals I've settled on working with (the ones that required the least $$ for me to get started) are printed on a paper-backed thin plastic in mirror image. After the ink dries the plastic is coated with an adhesive. Once that dries you cut it out, place it on the surface and use a soft pencil or something to rub over it.
For the latest test a second layer of adhesive was put on. It seems to be holding up very well. Next up will be to put two of 'em on a '56 Kodiak I need to refinish anyway and see how they work out.