The trick of course is to get the decal on in the right spot with no air bubbles. Make sure the area you’re applying the decal is clean. This is best accomplished with a mild soap and distilled water wash. Optionally you can clean it off with Acetone after soap and water. If you’re cleaning with alcohol and not washing after with distilled water, you could leave mineral residue behind unless you’re using 100% alcohol, which most alcohol in the medicine cabinet may not be.
Small decals can be directly applied with care. It’s sometimes helpful to use something like a squeegee to evenly press from one end to the other as you lay it down. If you do get bubbles, you can pop them with a pin or x-acto knife tip to let the air out.
To apply larger decals like number plate backgrounds, you can do it dry or you can use a 50/50 solution of pure alcohol and distilled water and “float” them on. You’ll need something like a small hard squeegee like this one. Plastic paint scrappers work great too. Spray the 50/50 water alcohol on your pre-cleaned and rinsed with distilled water panel. Set the decal on the panel and squeegee the water out from the center out. Not too hard or you may stretch, scratch or gouge the decal. You can use a hair dryer to heat up the decal to get it to stretch a bit to bend around. Sometimes the decals have to be sliced to easily fit onto a rounded style panel. You have to be careful not to stretch it past where you need it though, because it will only shrink back with heat so much. Once you’re done, don’t mess with it. Leave it to dry for several hours. The alcohol helps evaporate any remaining water and leaves you with a dry properly applied decal.
Here’s a short video showing both wet and dry decal application processes.