Place the 3/4-by-22 1/2-by-22 1/2-inch MDF panels parallel to each other on a work surface with the 3/4-by-22 1/2-inch edges resting flat. These are the sides of the modular shelving unit. Space the outside edges of the panels 24 inches apart.
Butt the 3/4-by-24-by-24-inch panels perpendicularly across the ends of the sides. These are the top and bottom panels of the modular shelving unit.
Drive trim head wood screws through the top and bottom panels into the ends of the side panels every 2 inches using a screw gun to fasten the assembly together.
Stand the assembly upright. Brush an even coat of primer onto the assembly with a paintbrush. Allow the primer to dry for at least 4 hours.
Apply paint over the primer coat with a paintbrush. Allow it to dry for 4 hours and apply a second coat. Wait 24 hours after the final coat before using the unit.