I've had okay results with Testors sheets. Once printed, let the ink dry for a couple hours, then spray a thin coat of decal bonder over that, let dry awhile, once more light coat, let dry overnight, then you're ready to go. Alternatively, just follow the instructions on the package!
Sometimes it workds great, sometimes it doesn't. And, you can't print white, which is a big drawback. They make white decal paper, but then the whole decal is white (and not very white at that)
It is worth giving it a try.