Purchase an awesome vinyl decal! (Presumably you have, since you found your way here!)
Prepare the surface. Most of our decals should be applied in air temperatures 45F (7C) to 80F (26C); the surface should be within this range as well. Clean the surface with rubbing alcohol and a razor blade, and allow to dry thoroughly. DO NOT USE STANDARD GLASS CLEANER. It leaves a film that will keep your decal from adhering to the surface.
Prepare the decal. Use a flat tool, like a squeegee or a credit card, to rub the decal, from the clear side, starting at the center and moving out to the edges. This helps ensure your decal adheres to the application tape, rather than the backing.
Carefully peel the solid backing layer from the vinyl decal. Only the decal and the clear application tape should remain.
Apply the decal to the surface – the clear application tape will allow you to precisely position the design prior to actually sticking the design onto the surface. Use your squeegee or credit card to rub the decal again, through the clear tape, starting from the center and moving out to the edges of the design. The adhesive on the vinyl is pressure-sensitive; the harder you press, the better it will stick to the surface.
Peel the clear application tape away from the design, starting at one corner and pulling the tape carefully back on itself – NOT pulling it straight out from the surface.
For most car decals, the areas of vinyl will be small enough that bubbles will not be an issue. However, if you do encounter bubbles while applying the vinyl, use a sewing pin or safety pin to gently poke a hole in the bubble and use the squeegee or credit card to smooth the air pocket out.