Place a 22 1/2-inch 2x4 on a flat surface so it is resting on its 4-inch side. Place a 46-inch 2x4 next to it so the 46-inch piece is standing vertically on one end and the 4-inch side of the 46-inch board is touching one end of the 22 1/2-inch board, creating an L shape. Screw these pieces together by driving wood screws in a horizontal line spaced an inch apart through the 46-inch piece and into the 22 1/2 inch piece. Repeat this for the other 2x4s.
Place a 7 1/2-inch 2x4 next to the the top of the 46-inch piece so the top of the 7 1/2-inch piece is even with the top end of the 46-inch piece and it is parallel to the 22 1/2-inch piece--the piece will now look like a C. Screw this piece to the 46-inch piece by driving screws in a horizontal line spaced an inch apart through the 46-inch piece and into the ends of the 7 1/2-inch piece. Repeat this for the other L shape to create two Cs.
Place the 29-inch 2x4 between the 7 1/2-inch pieces so that the 2-inch side of the 29-inch 2x4 is even with the ends of the 7 1/2-inch 2x4s. Attach this piece to the 7 1/2-inch pieces by screwing flat brackets to the underside of the 7 1/2-inch pieces and then screwing the 29-inch piece into those brackets.
Place a 36-inch 2x4 on top of the open end of the 22 1/2-inch 2x4 so that it is sticking up vertically and its 4-inch side is even with the end of the 22 1/2 inch board. Screw this into place by driving screws through the bottom of the 22 1/2-inch board into the ends of the 36-inch board. Repeat this for the other 36-inch board on the remaining 22 1/2-inch board.
Place an 18-inch 2x4 on top of the 36-inch 2x4 so the ends of the longer boards are touching the 4-inch side of the 18-inch board. The 18-inch board should be parallel to the 22 1/2-inch board. Screw the 18-inch board to the 36-inch board by driving screws through the top of the 18inch board into ends of the 36 inch board. Repeat this on the other 36-inch 2x4.
Place the 36x15-inch plywood on top of the 18-inch pieces of wood to create a shelf for the monitor. Screw this piece to the 18-inch 2x4s by driving screws through the top of the plywood into the 18-inch pieces. Space your screws about 2 to 3 inches apart.
Measure 1 foot from the bottom of the stand toward the monitor shelf and make a mark. Place the 21x36-inch plywood inside the stand at this mark. Attach the shelf to the desk by drilling through the 2x4 legs into the edges of the plywood. Use at least 2 screws per leg. This is a shelf for your hard drive.
Screw in the plastic keyboard shelf in the open space in front of the monitor shelf.