There are several methods and techniques to do this.  This is only a suggestion for our customers, we take no responsibility if you destroy your decals in the process!

We recommend using a decal setting solution when placing decals on a Helmet.  These are available in any hobby shop. 

1) Cut out you decals as close to the decal as possible.

2) Wet down the area on the helmet  you plan to put the decal on with decal setting solution or water.
Soak the decals in water till the decal can easily slide off.

3) Carefully slide the decals off the paper and onto the helmet, being careful not to rip them.

4) Position properly, centered  under the air vent and brush lightly with decal setting solution, allow to dry and put on as many coats of decal setting solution as it takes to get it to set on to the surface of the helmet.  After the first round dries, I examine the decal and get a very sharp needle and poke any air bubbles. Then apply the next drop of solution directly to the tiny holes you just made. This will soak in and help draw the decal down into those areas.

5) Once the decal is positioned and dry, then apply as many coats of solution until the decal has conformed to the surface of the helmet and all the air bubbles are gone.

6) After the decal is dry and in place, you can spray with a Dull-Coat, for a painted on look.