Instructions in white.Commentary in blue.
Price: ~$20/car, cheaper if making multiple cars
Difficulty: Pinewood Derby
Time: 30-60 minutes/car, depending on tools used
Step 1: Tools/Materials
-band saw/jig saw/coping saw
-miter saw/hand saw
-drill press - a hand drill alone will not work; holes need to be perfectly square
-wood drill bit [6mm, 7/32", 15/64"] - may vary
-wood drill bit [8mm, 5/16"]
-combination wood file
-drum sander/belt sander/palm sander/random orbit sander
-sand paper [100 grit] - for the powered sander
-sand paper [300 grit]
-sand paper [600 grit]
-drafting tools [pencil, rule, compass/2D CAD software, printer]
-metric hex keys
-router bit [45°]
-small wood block [2" x 2" stock, 1.5" x 1.5" actual] - I chose poplar as it was the smoothest I could get.
-4 inline skate axles [8mm diameter]
-2 bolts [M6 x 1.0mm x 50mm] - may vary - you can use threaded rod if building multiple cars
-4 standard inline skate wheels - must accept standard 608 bearings
-8 bearings 
-4 washers [8mm, regular or split]
For the wood, I went to Lowes and picked out the smoothest length of poplar I could find. They have a tiny section in the back of the store that has nice wood stock, in Pine, Poplar, and Oak. The wood in this section is overpriced, but one car doesn't require much. If you have an actual lumber supplier, go there and pick out something nice instead. Also, do you research and make sure the wood is food-safe, since these cars are intended for young children. Obviously, don't use treated lumber or any kind of particleboard.
The tires for the car are actually inline skate wheels, with the standard 608 bearings. I got mine from ebay, and they came with bearings. Most wheels, however, will come without bearings. You will need two bearings for each wheel, and can find them here. The bearings won't be supporting any substantial weight, so you don't need to use expensive, high precision bearings.
One of the problems that I found with Make's car was that the axles consist of hex bolts and are secured with lock nuts. This leaves big chunks of metal sticking out from the sides of the car. If kids are rolling these around the house, there is a big potential for scratched walls and table legs. Solution: use 8mm inline skate axles and a threaded rod as the axle. Many inline skate axles feature flat heads with Allen slots. This leaves a very small profile and no sharp edges. I got my axles here. Yes, I have extensively searched the usual online suppliers(McMaster-Carr, MSCDirect, Grainger, etc.) for sex bolts of the proper dimensions, but I couldn't find what I needed. Yes, I could machine my own bolt barrels and cut flathead slots in the heads, but the inline skate axles saved a lot of time.
For the threaded rod, I took an axle to Lowes and found that the axles are threaded at M6 x 1.0mm. This may be different depending on what axles you use. Because big box hardware stores in the U.S. don't stock metric threaded rod, I bought two M6 x 1.0mm x 50mm bolts and then removed the heads with a hack saw. On a side note, can you believe that Lowes no longer carries tap and die sets? They also suck at stocking their nut and bolt drawers; pretty soon I'll have to order everything from the internet.
Step 2: Pattern
The most important thing to consider is hole spacing. There must be at least 3cm between the wheels to allow for easy gripping of the car. Use this equation:
Minimum Distance Between Hole Centers = 3cm + Wheel Diameter
For example, I used 77mm wheels:
MDBHC = 3cm + 7.7cm = 10.7cm
So the minimum was 10.7cm. I ended up going with 12cm for aesthetic purposes.
The next thing to consider is ease of manufacture. Know the capabilities of your tools. If you can't cut chamfers in a safe/repeatable manner, then you should exclude them from your design. If making multiple cars, you want to minimize the number of cuts and plan for the kinds of curves that your saw can handle. Some saw blades may have large minimum radii, so do some test cuts to figure out what your options are. My design has a small concave radius at the tail that I ended up shaping mostly by hand. I suggest not altering the width of the wood stock in the areas around the axle holes. These areas need to be perfectly square.
The last thing to consider is front and rear protrusions. If these cars are to be given children, then the front of the car body should not protrude past the front of the wheels. This way, if the car is rolled at a wall, only the soft rubber wheels will touch.
A key feature of my design is that, on any flat surface, the car can not be oriented such that the wooden body touches that surface, even upside-down. This protects both the finish on the car and the paint on the walls.
Now you'll want to either print out and glue your pattern's side and top profiles, or draw them directly onto the wood.
Because my pattern is simple and I only made 6 cars, I ended up just drawing my pattern right onto the wood. If I were making 10 or more cars, I would draft my pattern in AutoCAD and print out copies.
Step 3: Cut a Block
Use a miter saw/hand saw to cut off the patterned section of wood, leaving a little extra. Nothing fancy here.
I cut a lot of blanks before applying the pattern. Doesn't really make a difference how you do it.
Step 4: Drill
First, you will need to drill the holes for the axles. This is the smaller of the two diameters, and it will go all the way through the block. IMPORTANT: to ensure that all four wheels of the car touch at the same time, you must drill both axle holes in the same orientation. That means drill the first hole with the front of the car facing left, then slide the block left and drill the second hole, all the with the block facing the same direction. This way, if your drill press is not perfectly square, the error won't compound and skew the axles. If you use this method, any small error in the drill angle won't matter.
I didn't do this with the first car I made and it turned out wobbly.
Next, drill the holes for the inline skate axles(8mm). These only need to be deep enough to make a clearance for the small length of inline skate axle still exposed when a wheel and washer are stacked on. This depth doesn't need to be horribly precise, just get it within 2 or 3mm. Use the same method as with the first holes to keep everything square.
Step 5: Cut the Side Profile
For the best results, use a band saw with a thin blade. If your pattern requires that you cut off the top of your wood block, be sure to keep the scrap and tape it back on to cut the top profile.
Step 6: Cut the Top Profile
Step 7: Shape to Taste
Cut out any other angles that may be in your pattern, and then roughly file the edges to shape the car and get rid of tool marks.
My pattern has two chamfers, which I cut with a band saw that has a rotating table. This is not the easiest thing to do and, considering I don't have a rip fence, not very accurate/repeatable. I got the hang of it but still had to square up the cuts with a file and electric sander. A router table with a 45° chamfer bit and a fence would have made it much easier.
Step 8: Sand
Take the car to an electric sander and get rid of any marks left by the saw and file. You can also smooth over any curves. IMPORTANT: don't sand the areas around the axle holes using a heavy grit! These need to remain square. You can sand these with the finer grits, but these areas won't be very visible. Sand all exposed surfaces with the 300 grit and then 600 grit sandpaper.
I only have a random orbit sander, so I clamped my cars in a vise with wooden jaws and sanded it that way. The vise blocks much of the car's surface, so I had to keep repositioning the car. A belt or drum sander would be much faster.
Step 9: Brand(optional)
I chose to brand my initials into all of the cars that I made, on the belly of each car. I just bent a piece of sheet steel and got it glowing with a propane torch.
Step 10: Apply Finish(optional)
If these are to be given to young children, you will want to use a food-safe finish on the car. You can also just leave the car bare.
Step 11: Assemble
With a hack saw, cut the heads off of the two bolts. Then assemble the car. There will likely be some slack between the washers and the car body. Record the length of this slack. Next, disassemble the car and cut off the recorded length of slack from each threaded rod. You can now use theadlocker/JBWeld on the threads to permanently reassemble the car.
I really like this project. I think the end result is very professional and I could see these in a classic toy store. Ideally, I would like to use all stainless steel hardware for the build. I would also like to try the smaller skateboard and roller skate wheels.
FEEDBACK: Does the color-coding make this tutorial more clear or less? Let me know in the comments.