Apply the art to one side, and then lay it face down on a hard, smooth, clean surface- like a fresh sheet of smooth wood, or a really nice floor. Make sure it lays flat all the way around- you don't want any gaps between the art and the hard surface you're using. Run a razor knife (Exacto or box cutter or carpet knife, whatever) around the edge of the cab's side. The razor will likely scratch the hard surface, so if it's a floor, make sure it doesn't belong to somebody that can kick your ass. Repeat for side two.
I would definately NOT cut the art to size first, and then try to apply it. You'll never get it straight.