These decals are printed on TANGO PAPA decal paper; sometimes the film is very thin. My artwork has been clear-coated to protect the art and to make the decal a bit thicker. I strongly recommend that you clear-coat them again, and even then, be careful when handling them.
Use an airbrush and spray the decals while still on the backing paper with TAMIYA or GUNZE SANGYO clear gloss acrylic paint.
WARNING: Enamel or lacquer based products will likely damage the art.
The decals are printed on clear film which covers the whole sheet of paper. Using a hobby knife, press softly and cut the film, all around the art, as close to the art as you want. You dont have to cut all the way through the paper; just press enough to cut through the film on the paper. This is called "scoring".
Now, using scissors, cut the individual pieces apart from each other. Cut between your scored lines. The shearing force of the scissors can cause cracking in the art or in the film which is why scoring is important. Scoring also eliminates the excess film from around the art.
Hold the piece of decal paper with tweezers and submerse it for only a few seconds in a bowl of lukewarm water. Really, 5 to 10 seconds is enough. Then pull the decal out and let it sit on your work area for 10 to 20 seconds. If the paper curls do NOT try to flatten it. Let the paper straighten on its own. Rushing it will cause the film (and the art) to shatter. Remove any excess film from around the decal with tweezers.
Wet the surface of the model with water from the bowl. You can use a setting solution if you want. Some solutions are very strong; they can attack the paint and the decal. Remember, these decals are very thin so they should settle down onto practically any surface. Keep the surface of the model wet.
Place the decal and the paper on the model close to where you want the decal positioned. Using a brush or sponge or cotton swab, slide or push the decal off the paper onto the model. Remove the paper. If you try to lift, peel, or pull the decal off the paper it could curl or tear.
Keep the surface of the model wet. Slide the decal around until it is positioned where you want it.
I know this next step breaks all the rules of decal application but I do it and it works for me: Gently rock or roll a piece of foam / dense sponge across the decal. This soaks up the excess water and helps push the decal onto the surface, squeezing out any bubbles.
Let the decal dry. Dont touch it for several hours. After it is completely dry (a day or two) you can airbrush over it to seal it and bond to the paint and to the model.