I have printed a number of custom decals. Your choice of papers are:
1. Clear backing for inkjet (good for dark images on light backgrounds)
2. Clear backing for laser
3. White backing for inkjet (needed if you either have a dark background/light image or any white in design)
4. White backing for laser
Just choose what you need based on your design and the printer you have. The hardest part is having to cut right up to the edges of your design when using white backing. Either inkjet or laser MUST be sealed with the sealer. Testors is a great sealer, but you can use clear acrylic sprayed lightly in a pinch.
Use some decal solvent when applying and I have had better luck with these than with decals that actually come with the models. Here is the paper I like best, since it is pretty cheap and can be found in some Hobby Stores in 1 and 3 sheet packs:
http://www.decalpaper.com/