NS:
Randy was right but a bit brief. First decals(Can't have to many decals) need a smooth place to settle into. Zo you need to cover the model with Future or a Gloss coat of your choice after paint has throughly dried...Get decal loose on backing paper in solution of water with a drop of detergent in it. Put a drop of Set,think of settling down, on model and slide decal onto it. Use a damp Q-tip or lint free rag. to position decal, plane must be in horizontial position of course. Pat down gently and wick away any excess water or setting solution. add a drop of setting solution to top of decal and after being sure its in right place, leave it alone for 24 hours. If all went well you should be able to gently clean area around decal and after everything is dry seal with final finish coat of semi gloss or what ever you like.This is basic approach. Thick decals, silvering, nonadhereance, & bubbles require different methods . Good luck Chief John