Measure the dimensions of the shelf on a piece of 3/4-inch-thick wood. Start at the edge and measure down 4 feet, then measure across 4 inches. Connect the dots with a straightedge to form the long rectangular shape. Cut the board along the lines, using a Skil saw.
Smooth the board with 220-grit sandpaper. Smooth the edges and the top by hand to avoid removing too much of the wood.
Paint or varnish the board. Apply with a paint brush using long, smooth strokes. Match the color or finish to the surrounding closet. Allow to dry for 24 hours.
Install L-brackets on the closet wall. Measure from the floor to the desired spot on all three sides of the closet. Make two marks on each wall and connect with a straightedge to form a line. Use a drill to place the L-brackets on the wall along the line. Use one bracket on each of the ends and four evenly spaced brackets along the back. Use the screws that come with the L-brackets for best results.
Place the 4-inch by 4-foot board on top of the L-brackets. Use a drill to insert screws upward into the shelf to secure it in place.