If the decal has a flat finish, cover it with painter's tape and just clean it with soap and water. There are QD spray that's designed to clean and provide UV protection without making the decal shiny.
If the decal is glossy, you should be able to wax right over it. Just be careful about wax build up at the edges. Wash with soap and water should be all the prep that's needed. I personally would not clay bar or polish over the decal. I don't think the finish is as tough as the clear coat on the car and may scratches more easily.
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