Hi, cnq, I never have made my own, myself, but I know of tutorials and discussions at the scale modeling sites. You might want to have a look at these:
Fine Scale Modeler:
http://cs.finescale.com/f/
Aircraft Resource Center:
http://www.aircraftresourcecenter.com
Modeling Madness:
http://www.modelingmadness.com
Agape Modeling:
http://agapemodels.com/forum
It's supposed to be relatively easy, creating your images in an imaging program, and then printing them on decal paper. It used to be necessary to use an ALPS printer, but I think that's no longer the case.
Hope that helps, prost!
Brad