Most inkjet printers use ink that is soluable in water. This is not good for waterslide decals, so you need to overcoat the printed decal before using it. I overcoat in testors glosscoat, but there are other clear overcoats that work: enamel, acrylic spray, and polyurethane, among them. If you have a glossy printing paper, such as photo paper, you can test your printer ink by printing something, then spraying it with the clear coat. If no photo paper, by as small a package of decal paper as you can.
If it does not run or show fringing at the edges, you are okay. If it runs, you may need a different printer. The printer I used to have had a lousy ink, so I bought a cheap forty buck printer just for decals. I made sure I could return it if it would not work for decals, but that cheapy printer worked fine. I have since bought a more expensive printer for photo printing, that also works fine.
To make a decal with light colors or white, you need white decal paper. If the decal you need is large, and a simple shape, print with a dark background and cut out the light decal with a hobby knife. If the design is complex, then you need to match the background color very closely, and this is an acquired skill. That background matching problem means that making a light colored decal to print over camouflage is a super challenge and may not be practical.
Like learning any new technique, start with a simple project, like black lettering or numbers. This is a pretty easy job.