Cut the 1/2-inch by 6-inch, and 1/2-inch by 3-inch lumber planks to equal lengths with a circular saw. The length can be any size you desire. To hold at least four pairs of shoes, cut the lumber to 36 inches.
Sand both the 1/2-inch by 6-inch lumber planks and the 1/2-inch by 3-inch lumber planks with medium grit sandpaper to make them smooth and get rid of splinters.
Blow off the lumber planks with compressed air, then paint the four lumber planks any color of you like with a paintbrush. Allow to dry for three to four hours.
Waterproof the planks by applying a coat of polyurethane to each plank with a clean paintbrush. Allow the polyurethane to dry overnight.
Gently sand each plank with ultra-fine sandpaper to make it smooth. Then, fasten an L-bracket to the underside of the 1/2-inch by 3-inch planks with screws and a screw gun.
Connect the larger planks to the smaller planks by fastening the L-brackets to the back of the 1/2-inch by 6-inch lumber planks. You'll now have a horizontal U shape, with the narrow planks on top and the deeper planks on the bottom. Attach the shoe rack to a wall using a screw gun and screws.