In its simplest form, a tow bar is simply a straight piece of rigid material that is attached from the towing vehicle to the vehicle being towed. The idea is that your car or trailer can be towed while not changing the distance to the car that is towing.
You can make a very simple tow bar by simply cutting a 6-foot piece of 4-by-4 or even a tree trunk with a 4-inch diameter. Drill a 1-inch hole in each end, and take a 10-foot piece of 1-inch nylon cord and run it through the holes. Wrap the cord several times around the piece; tie it off and then tie it securely to the bumpers or toe hitches of each car or trailer. This apparatus should only be used if nothing else is available.
More Sophisticated Bar
Take a 1-inch piece of galvanised pipe and two u-clamps mounted on 7/8-inch bolts. Slide the 7/8-inch bolt on the clamp into the pipe and weld around the end. You can now attach the u-bolts to the towing hitches on each car with a 3/4-inch bolt. If you want a more structurally sound bar, you can weld a triangular support system.
Before towing make sure that your bar is well in place. You should use the wooden bar only in an emergency. The galvanised pipe bar is stronger; be sure you have welded it securely.