Arrange side boards parallel to each other so they are resting on their long thin edges on the work space. Hold both boards upright with the help of an assistant and position the top board perpendicular to them with the ends of both side boards butting into the side of the top board. Position the side boards so that they are flush with the edge of the top board.
Predrill three evenly spaced holes into the top board and through the end of each side board for six total holes. Choose a drill bit that possesses a slightly smaller diameter than the wood screws used in the project. Attach the top board to the side boards by drilling the two-inch wood screws through the holes. Repeat this process to attach the bottom board.
Set the rear board on top of the assembled storage shelf frame, and adjust it until the edges of the rear board are flush with all the edges of the assembled storage shelf frame. Predrill holes every 12 inches around the edge of the rear board and into the frame. Attach the rear board to the frame with wood screws.
Stand the unit upright and measure from the top of the bottom board on the inside of the unit up a distance of nine inches. Mark this spot and measure up another nine inches. Mark this spot and measure another nine inches for a total of three markings. Repeat for the opposite side board.
Drill two holes at the level of each marking, using a drill bit the same diameter as the pegs while being careful not to break through the outer surface of the side boards. Insert pegs in all the predrilled holes and set the shelves on the pegs.