Place 1/2-inch plywood on a table or workbench, then measure and mark cut lines with a tape measure and pencil. The shelf can be as wide and deep as you need for your particular space.
Put on eye protection and cut the 1/2-inch plywood to size using a circular saw.
Sand the shelf smooth with medium-grit sandpaper, then brush away any sawdust from the sanding with a paintbrush.
Paint your basement shelf with a paint color of your choosing and let dry for three hours. Apply a second coat of paint if necessary, and allow to dry completely.
Apply a coat of polyurethane with a paintbrush to protect the paint and seal the plywood shelf. Allow the polyurethane to dry overnight.
Attach L-brackets to the underside of your custom basement shelf using stainless-steel screws and a screw gun.
Hold the shelf against the wall, with the L-brackets pointing downward, and mark the location of the holes in the brackets with a marker. Set the shelf aside momentarily.
Fit a masonry bit into a drill and drill holes into the wall through the marks. Then fasten the shelf to the basement wall with screws and a screw gun.