Use the tape measure and pencil to measure and mark a line 2 feet from one of the short ends of the plywood. Use the straight edge or level to extend the line across the width of the plywood. Cut along the marked line with a circular saw. Set this piece aside. It is the top of the desk.
Measure and mark a line 1.5 feet from one of the long sides of the remainder of the plywood. This piece should be 6 feet long by 1.5 feet wide. Cut along the marked line. Now divide this same piece into six 1-foot pieces. You will end up with six pieces that are 1 foot wide by 1.5 feet long. These are the shelves. Set them aside.
The remainder of the plywood is now 6 feet long and 2.5 feet wide. Measure and mark in increments of 1.5 feet. Cut along the marked lines. You should now have four pieces that are 1.5 feet wide by 2.5 feet tall. These are the side pieces. You will use two side pieces for each side unit.
Place two of the side pieces side by side, facing the same direction. The 2foot, 6-inch length is the height of the side, and the 2-foot piece is the width. From the bottom of each side piece, measure up 3 inches and mark along the width of each piece. This is the line for the bottom shelf.
Squeeze a line of wood glue along the marked line of two of the side pieces. Place a 1-foot wide shelf piece on the glue. Make sure the shelf is turned the right way. The length of the shelf should match up with the width of the side piece. Use the tape measure to keep it straight. Repeat with the second piece. Allow the glue to set for a minute, then drill several wood screws from the outside of the side piece into the thickness of the shelf. Evenly space out the screws for a neat appearance.
Decide where you want the other shelf to go. You may want to measure what you plan to store on the shelf to allow clearance. Mark the desired position on the side piece and repeat the same procedure in the previous step to attach the shelves to the two side pieces. Attach the third and final shelf piece at the top of the two side pieces. This will serve as the top of the side unit. You will drill through this piece when attaching the desk top.
Lay the two side pieces flat so that the long sides of the shelves are facing up. Apply glue to the long edges of each shelf. Take one of the remaining side pieces and gently place it on top of the long sides of the shelf. Make sure to have the height and width running in the same direction as the side piece below it. Repeat the same procedure with the last side piece. Secure with wood screws. You now have two completed side units.
Position the two side units side by side, 2 feet apart. Put the desk top on the two side units. Adjust the side units as necessary to fit evenly under the desk top. Carefully remove the top and apply wood glue along the top edge of each side unit. Replace the desk top. Secure the top with wood screws from the underside of the desk through the top of the side units. Sand all the edges and enjoy your new computer desk.