Follow the instructions below:
1. Clean surface to remove any dirt, grime, etc.
2. Without removing the paper backing, position decal on window exactly where you want it.
3. Once the decal is in position, place a piece of masking tape along the top edge to hold the decal in place.
4. The decal should be sitting on whichever surface like a flap. Lift up the decal (A) and remove the paper backing (B).
5. With the backing paper completely removed from the decal & transfer tape, gently lower the decal with the transfer tape back down to the surface of the glass and rub it down lightly with your hand.
6. Take a squeegee or credit card and firmly rub the decal down to your surface. The transfer tape should still be on the decal. This allows you to rub the decal down without scratching or damaging the decal.
7. Finally, peel off the transfer tape and masking tape gently. The decal will adhere to the surface much more aggressively than the transfer tape. The transfer tape should lift easily leaving behind no sticky residue.
8. Your decal installation is complete. If there are bubbles present under the decal that cannot be removed by working them to the edge, a small needle or pin can be used to puncture the bubble and remove the air. A small pin hole will not be seen after the air bubble has been worked out.
OR,...here's a video: