In advance: Make cookies. Prepare and roll out dough. Cut cookies using car cutter from set. Bake and cool cookies. Outline lower car body using tip 3 and full-strength red color flow; flow in using thinned icing. Let dry on waxed paper-covered boards.
Decorate using full-strength color flow. Pipe in tip 3 white background areas for windshield and front grill; pat smooth with finger dipped in cornstarch. Use gray to pipe in tip 3 grill interior (pat smooth), dot lights, windshield and steering wheel outlines and zigzag convertible top. Use black to pipe tip 1 lattice grillwork, tip 3 light border, tip 4 outline tires and arm. Pipe tip 4 dot glove; flatten with finger. Pipe tip 1 outline finger details. Pipe tip 3 outline hubcaps and dot axle. Pipe tip 3 front bumper. Attach icing decoration with icing dot.