Write an article based on this "Clean the area. Do not use any cleaner that leaves a film or oil on the car. Allow the cleaned area to dry completely. Peel off the beginning of the backing."
Once you have the area the decal will be applied determined, use a soft cloth and a mild cleaner on the surface of the car.  Rubbing alcohol will clean the area well and evaporate quickly, making it a common choice.  Do not simply spray the cleaner onto the car or it may soak the masking tape you’ve used as a guide. Apply cleaner to a rag and wipe where the decal will stick to the car. Some cleaners like Windex or spray on cleaner-waxes leave a residue intended to help maintain shine.  This residue will prevent the decal from sticking properly to the vehicle.  Rubbing alcohol or basic car wash soap both will clean the area sufficiently. Be sure to rinse the area thoroughly if you use car wash soap. Decals won’t stick as well to a wet surface, so either dry the area you cleaned with a towel or allow it to air dry completely before moving on.  If you opt to use a towel, use a chamois cloth to avoid damaging the paint. Wait for the surface to be completely dry before applying the decal. Depending on the size of the decal, you may choose to remove the entire backing or just a portion of it at a time.  If the decal is larger than your forearm, you may want to peel off the backing in thirds to make application more manageable.  Decals that are only a few inches long can have the backing completely removed. Only peel off a bit of the backing at a time for larger decals such as ones that will stretch the length of the vehicle.