Place the 46-inch-long boards parallel to each other with the outside edges 48 inches apart on a work surface. The edges should be flat on the work surface. These are the sides of the shelving unit.
Position the two 24-inch board across the ends of the sides to form a rectangle. The edges should be flat against the work surface. These are the top and bottom boards of the shelving unit.
Drive three evenly spaced wood screws through the top and bottom boards into the ends of the sides at each joint using a screw gun.
Insert one 221/2-inch board between the sides so that they are parallel to the top and bottom boards. These are the shelves. These edges should be flat against the work surface. The ends of the boards should be flush with the inside face of the sides. Space one shelf 12 inches from the inside edge of the top and one shelf 12 inches from the inside edge of the bottom.
Drive three evenly spaced wood screws through the outside of the sides into the ends of the shelves at each joint.
Paint the assembly with a paintbrush and wood finish. Allow at least four hours between coats if you decide to apply more than one. Wait at least 72 hours before placing anything onto the shelves.