Ferris, You don't cut anything on the truck. You apply it carefully in the shape you desire around the outside edges of the letters, in this case. Use the decals you have as templates to cut what is essentially a "negative" of the decal out of the masking material.
You can use "fine line" tape for the detail curves or if you have a sign shop around that makes the vinyl decals, they have a "masking material" that does not stick permanently and they cut with like a CAD printer so you can get a perfect masking decal.
