Windex or water, either will work. Have a few paper towels handy. Wet the helmet, wet the decal. Put it on and it will be like hot butter sliding around on your helmet. Get the decals into place where you like them. Then, using your thumbs start "squeegie-ing" or pressing the water out of the decals. Start in the center of the decal and with your thumbs push outwards to get out the liquid from under the decal. Dab it with your paper towel to absorb the liquid. DO NOT panic if your decals are so wet they they actually curl back onto your thumbs, its typical. Just keep thumbing away and eventually as you thumb and dab they will adhere. Once they start to adhere, get out any bubbles and that's it. I just did a helmet 2 hours ago with this method and it rocks, I used straight water and it worked fine. FX helmet took me about 8 minutes for all the decals.