Choose a car to model. You may not be accurately representing the exact dimensions and expressing every detail in photographic quality, but having an idea to base your design on is a good first step. Find a good profile or straight front view, depending on how it will be used, of the car in question. Create a simple coloring-book-style outline drawing of your car on white paper with a crisp black outline.
Project the image of the car outline in the size you want onto a sheet of 3/4-inch plywood with an opaque projector, available at an art-supply store, and use a marker to draw it on the plywood. These projectors require complete darkness and can use a paper image, photograph or book to create a projection.
Cut the profile out of the plywood using a jigsaw. Move the saw at a steady rate and use a fine tooth blade at full speed for a clean cut with a minimum of splintering. Cut out the doors and windows only if you will need to see inside.
Sand the cutout with a 100-grit belt on a belt sander to prep the surface for painting. Brush on the paint with a fine-bristle brush, blocking in all of the major colors first, as you would in a color book. Use an artist brush and mix a small amount of each color used on the car in paper cups. Mix one cup of each color with one part white paint to two parts color, and a cup with black in the same proportions.
Apply the lightened version of each color to that color on the car in areas where the street lights or sun might cause a highlight, or reflection. Apply it lightly, allowing the base color to show through. Do the opposite with the darker versions of your colors, applying them wherever the street lights or sun would cause shadows. This will help give the car dimension.
Attach the cutout to the front or side of a rolling platform, or wagon, which most theaters and schools have in their inventory, using 6-inch L brackets. Add a bench or chairs for the actors. Roll the car in during a blackout, when the lights are off, and remove it the same way.