With adhesive backed decals I've used both of these methods with sucess. The first is the windex method you mentioned. Some people spray the windex on the surface but I spray mine on the decal itself. Then place it into position and then when it's in place, use a credit card or small squeegy to get all the airbubbles and excess windex out. Then just let it dry overnite. The other method is basically the same proceedure except instead of using windex, I used warm water in a bowl with a couple of drops of liquid detergent.