Your boy should begin by selecting an overall design. Help him develop a design from magazine or catalog pictures, or simple sketches. The shape of the car does not strongly affect final performance. Generally a simple concept is best, such as the VW Beatle shape, or variations of the popular wedge shape. Draw an outline of the design on the block. The parent should rough cut the block, especially if power tools are involved, and perhaps do some of the heavy sanding. Leave plenty of wood around the location of the axles. The boy should finish with the fine sanding.
Weigh the car, wheels, axles, trim, adding sufficient weight to obtain a total of five (5) ounces. Usually, this can be done at the Pack work event. If you prefer, you can drill holes into the body and insert solder, fish line sinkers, nails, or attach a small metal plate. Since solder shrinks when it cools, you will need a way to keep the solder plug from falling out of the car. Be sure to remove any part of the weight that hangs below the bottom of the car, otherwise the car may rub against the track and will not run well.
Fill any nicks or holes with plastic wood, fine sand again, and you are ready to paint. An enormous variety of paints and finishes exist, check the hobby shop for information about what is available. Discuss some possible finishes with your boy, then let him paint his car in the colors that he chooses. Remember to paint the bottom of the car for a really nice appearance.
Mounting the wheels is the most important element of good performance. Using a piece of emery cloth or an emery fingernail file, remove the small burr on each of the wheels. A good technique is to slowly turn the wheel in the chuck of a power drill, then lightly hold the emery surface against the wheel. Your boy can share this, but be careful because it is very easy to destroy the wheel by heating the plastic. It is also helpful to file off any burr on the axles, then polish the axles with steel wool or emery cloth.
(Note: The wheels now being supplied are much better then when I started building cars. Most of the time, you will not have to sand the wheels.)
Do not taper the flat tread of the wheel or the hub, do not use speed wheels, washers, or speed axles. Any car which uses these advantages will be disqualified.
Carefully insert the axles into the slots provided in the car with the axles as straight and level as possible. There is no good way to do this. Some people tap the axles in with a small hammer, some put the car into a vise and push the axle in by slowly closing the vise. Some people soap the axle before insertion. Adjust the axles to balance the weight on all four wheels, checking that the car rolls straight on a flat surface. Again, be cautious, too much adjustment of the axles will destroy the slot and prevent the car from being able to race.
(Note: Some people argue that a car will run faster if only three wheels touch the track. I have no idea whether this works.)
Finally, your boy can make his car just perfect by applying decals or special trim. Now, he is ready for an afternoon of tough competition and good fun at the Pinewood Derby.