Chocks physically prevent a wheel from moving forward or backward unintentionally. You can make chocks out of metal, wood or any other material that the wheel is unlikely to crush. You usually place chocks so that one is immediately in front of the wheel and one is behind it. One familiar form is the blocks placed around an aeroplane's wheels while it is on the ground. You can use chocks to keep a car from rolling while you are changing a tire.
Cut the 4-inch-by-4-inch lumber into two pieces that are 18 inches long. While 18 inches is wider than most automobile tires, the added length will allow you to place the chock around larger tires.
Place the two 4-by-4 pieces around one of your tires, one in front of the tire and one behind the tire. Measure the distance between the outside edges of the blocks. This is the width of the chock
Cut the 1-inch-by-4-inch lumber to a length equal to the width of the chock. Nail the 4-by-4 blocks to the ends of the 1-by-4.
Paint the chocks yellow for visibility, and as a reminder to remove the chocks before moving the vehicle.