Set two 1/2-by-12-by-24-inch boards upright on their small ends, parallel to each other. Hold these side boards in place with another person, and set a 1/2-by-12-by-36-inch top board across them. Adjust the alignment so the edges of the boards line up evenly, and drill four holes down through the top board into each side board.
Put 2-inch wood screws in those holes and completely fasten the boards together by tightening those screws with a drill. Repeat the process for another 1/2-by-12-by-36-inch board on the bottom of the unit.
Stand the unit on its side so that an open rectangle is facing upward. Position two 1/2-by-12-by-24-inch vertical support boards evenly within the frame, parallel to the side boards. Drill two holes through the top and bottom boards of the frame and into each end of the vertical support boards. Attach the boards to the frame with 2-inch wood screws.
Stand the unit upright. Insert 1/2-by-11 1/3-by-12-inch shelf boards into positions within the frame. Mark their locations on the side boards and the vertical support boards. Drill two holes into the side boards and support boards at these markings and put pegs in the holes. Set the shelves on top of the pegs.