Sketch a monster truck approximately 4-by-3 feet on a wall that is not blocked by furniture or above the bed. Use a photo reference to guide you. Draw huge all-terrain tires with a regular truck body. Consider the colors of the vehicle and add flying mud splats.
Paint the sketch of the monster truck with acrylic paints using artist paintbrushes. Allow it to dry for at least two hours. Add a second coat if necessary or touch up any areas that need more color. If you cannot paint a mural, use a monster truck wall decal instead.
Draw road signs on other walls, such as a stop sign and a monster truck parking sign. Paint them with the acrylic paints. Apply wall stickers if you do not want to paint handmade signs.
Move the furniture back into the room, if needed. Make the bed with monster truck bedding with pillowcases and sheets. Cover the bed with a monster truck comforter or duvet cover. Add throw pillows with monster trucks on them.
Hang a wooden shelf on a wall. Arrange a selection of monster truck toys on the shelf. Place a truck lamp on a desk or night table. Hang plain or neutral curtains that match the base color of the wall and the monster truck comforter.