Taken from http://www.fastwrx.com/howtoapvide.html
Hope it helps anyone who is not sure.
Applying vinyl decals isn't difficult, but if it's your first time we have some pointers that can lead to a quick and easy application.
The vinyl arrives in a three-layer sandwich. The decal will be visible through the backing paper, which resembles masking tape. There's also a protective layer over the sticky side of the decal. You'll need to remove that protective layer just before you apply the decal, but you'll leave the masking tape layer attached to the FRONT of the decal until it's fully applied. You'll apply both the decal and its masking tape backing to the car, then peel away the masking tape backing.
If the decal is large, we recommend measuring and making some masking tape guide marks on the car to make sure the decal goes on straight. If you're applying on the front or back of the car, measure from side to side to find the center point on both the car and the decal.
The vinyl will not stick as well to any oil or waxy surface. Use a little rubbing alcohol to clean the area where the decal will be applied. If you're applying the decal to glass, DO NOT use Windex or other ammonia-based cleaners; the ammonia interferes with the adhesive.
For small decals like the "Objects In Mirror" one, smooth the decal down onto the glass with your finger. For larger ones, use a credit card or other flat plastic as a squeegee to smooth the decal down against the surface of the car. Move slowly along the length of the decal, rubbing in an up and down direction (top to bottom) on long decals, and moving from one side to the other. If the decal is longer than a foot or so, stick the middle of the decal down first, then rub towards one end. Then go BACK to the middle and rub towards the opposite end. This should reduce the chance of your decal wrinkling.
The last step is to CAREFULLY remove the masking tape backing. Sometimes the decal will want to stick to the backing more than the car (good cleaning with alcohol will reduce the chances of this). If this happens, use your finger to anchor the decal in place while you lift the decal off.