The Quick and easy way to apply these decals is to peel back the paper backer just enough to expose about a quarter inch of the decal. Make sure the window is clean. Position the decal and lightly touch the exposed portion to the glass. You'll need a squeegee or similar with a smooth even edge. Using even pressure and starting where the decal is tacked to the glass, start working the decal onto the window, sliding the squeegee, while peeling back the paper backer. Try to do this in one movement. If you stop and lift the squeegee, you may get a thin line of bubbles. Peel of the transfer mask and your done. The draw back to this method is, once it's down, it's down for good.
Start with a clean window. Usually you would use a solution called Slip, but you can make your own out of dish soap and water. One good drop of dish soap in 1/2 gallon of water will do it. The dish soap (even though its a small amount) will make the water slippery and allow bubbles to be forced out from under the decal. Peel the paper backer off of the decal and get the adhesive good and wet with the solution. Place the decal onto the window and squeegee from the center out. You should be able to look through the window and see if there are any bubbles. If there is, work them out with the squeegee. Let the decal sit an hour or so to let the solution under the decal dry up a bit. Then peel off the transfer mask and your done. If you try to peel the transfer off to soon, you might pull the whole decal off. The nice thing about this method is that you can reposition the decal (before it's dry) if you don't like the position or accidentally put the decal on upside down (crazier things have happened).