Print them onto decal paper. I usually get my decal paper from Micro Mark. There are two versions, one with clear background and one that has a white background. The clear background is for dark colors to be applied to a light-colored background and this works well. Also, there is inkjet paper and laser paper. That should be self-explanatory.
White and light colored decals are much harder to do. Unless you are a wizard in cutting out the decals you are usually left with a white border. The way to fix this is to replace the background color of the decal sheet after scanning with the background color the decals will be applied over. This takes a pretty good knowledge of your photo-editing software, and a color-calibrated scanner and printer, so it is not nearly as easy as dark colored decals.
After printing, you need to seal (waterproof) the decals, as most inkjet inks are water soluble. For matt finish, I use Testors Dullcoat. Glossy decals are now harder. Testors Glosscoat used to be a fast drying lacquer and worked fine. Glosscoat has changed- now is a slow drying stuff (I think it may be enamel) and will dissolve the ink! I am still looking for a sealer coat material that works well (I am looking for a generic clear spray can lacquer).
I do have a black laser printer and for black-only decals I keep some clear laser decal paper. It still needs to be sealed, but is much more waterproof than inkjet ink and glosscoat works okay.