Its fairly simple. You need a printer and decal paper. You can do it with an ink jet printer or a laser, you just have to make sure you get decal paper for that printer. If you print with an inkjet you have to apply a clear fixative to the decal after printing to bind the ink to the decal film.
You can usually find decal paper/film at a good hobby shop, usually Testor brand. There are also several places on the Internet to buy from, including ebay. Decal paper comes in clear or white.
You have to remember that you cannot print white with a printer, so you have to take that into account when selecting a graphic and paper.
Other than that it is a breeze.
If you don't want to mess with it, I print decals as a service for $10 for a 5x8 sheet.
I'll be glad to help you in any way I can. PM me if you need any help.