Dump trucks have a few distinctive visual components -- a cab, wheels, headlights, and, of course, the hinged, open-top bed that does the dumping. These are all easily represented by geometric shapes you can create with a few sacks or tubes of colored frosting and some candies. Use simplified, stylized shapes and details for cookies, because you'll be repeating each figure for the entire batch. if you don't want to mix up your own frosting, you can buy it in frosting in ready-to-pipe tubes, which, conveniently enough, typically come with thin, round nozzles -- the only nozzle shape you need to decorate dump truck cookies.
Cut the pointed tip from the piping bag, also known as a pastry bag, with a pair of scissors as needed. Attach the nozzle to the bag, using a coupler as needed. Spoon the black frosting into the bag, filling the bag half full. Twist the bag close and press lightly until the frosting begins to flow. Repeat the process to assemble red, white and yellow frosting bags.
Press on the black frosting bag gently to pipe the outline of a front and back wheel on the truck. Pipe a line from the middle of the wheel to the edge of the wheel with black frosting. Continue to pipe lines in this manner, like spokes on a bicycle wheel, to fill each wheel shape. Press a red candy into the center of each wheel.
Outline the shape of the front of the dump truck, or cab, with red frosting. Outline a square shape in the middle of the cab for a door window. Fill the cab shape with vertical lines of red frosting, leaving the window area plain. Place a yellow candy on the front of the truck as a headlight.
Outline the inside of the window with white frosting. Fill the window shape with horizontal lines of white frosting.
Outline the shape of the back of the dump truck, or bed, with yellow frosting. Fill the bed shape with horizontal lines of yellow frosting. Place a red candy on the back of the truck cookie as a tail light.