Lay down drop cloths in your workspace.
Construct the two nine-cube shelving units according to the instructions. Place them parallel to one another on the floor, spaced a few feet apart.
Lay your MDF top flat across the two shelving units. Adjust the distance between the units so the corners of the MDF top line up with the corners of the shelves --- the shelves should be directly under either end of the top.
Attach the top to the shelving units by screwing them together with a power drill. First, drill a hole with a bit the same size of the screw head, and then drill the screw into that hole. While screw sizes can vary, depending on the thickness of your wood and shelves, you should use screws at least 2 inches long.
Lightly sand the shelving units to strip away some of their smooth finish. This helps the primer adhere.
Paint the desk with a coat of primer. If you are painting your desk a color other than white, you may mix some of the color in with your primer; this can save you from painting extra coats of color later.
Use a roller to give your desk an even coat of paint after the primer dries. Wait for your paint to dry, and then give it additional coats as necessary for an even color. Use thin, even coats for the best results.
Coat your desk with varnish or polyurethane finish to protect it.