Make a balsa wood squeege and use windex or water with a little soap in it. Once the decal is in place, use a soft cloth to squeeze out most of the water and wipe up. Then use the squeegee to force out the rest of the water. Always work from the center out and take your time. If the wing is not fully sheeted, then the decals won't want to stay down as you squeegee, but eventually when you get enough water out they'll start to stay down. Give them a good day or more to dry before you fly the plane - especially if it has a glow engine.

Also, be sure to sand the squeegee smooth so that you don't scratch the decals.