Okay, first, very big, issue is, are these dark colored decals to use on a light colored finish, or do the decals have to be or have white or light colored colors in them. The first are easy to do, the other is quite hard.
The first uses clear decal paper. I use the stuff from Micro Mark. I use Paint Shop Pro software, but Adobe Photoshop Elements is more widely known and you can get help with it more easily. However, both of these programs are somewhat overkill for most decals. Almost any graphics software can be used, even CAD software.
You can proceed in two ways- one is to completely do the graphics yourself with the software, the second is to use a photo or existing graphics and clean it up and resize it in the graphics software.
Now, inkjet printers do not print white, and when you print light colors the result lacks opacity, so for decals containing white or very light colors, especially to go over a dark background, you must use white decal paper. But then you must cut out the resulting decal perfectly, else you end up with a white border. You can ease this task by setting the background color in the software to the background color of the model. This eliminates using the decals over a camouflage area. Also, color matching is not easy to do with your computer and graphics software- you need to be pretty much into graphics software.
Finally, many inks are water soluble, so you must overcoat the finished printout with a waterproof clear coating before you immerse the decal in water.
Now, some folks will advise you to use an Alps printer, which can print white. But the Alps printers are quite difficult to find, and so are the ink/pigment ribbons.