Draw a line diagonally across an 18-inch square piece of plywood.
Cut the square into two triangular pieces using a handsaw or a circular saw, cutting along the pencil line you drew.
Smooth off the sawed edges with medium-grade sandpaper.
Paint the triangles with two coats of paint to match your walls. Allow the paint to dry thoroughly.
Place one shelf piece against the wall in the corner with the longest edge facing out. Place a level on the shelf to get the most level and even shelf. Draw a line on the wall to mark the underside of both sides of the shelf corner.
Measure out 8 inches from the corner along each wall and mark this spot. This is where you will attach the brackets. Follow the manufacturer's directions for the mounting hardware to attach toggles or anchor hardware to the wall. Attach the brackets to this hardware.
Lay the shelf on the brackets and attach it by drilling 1/2-inch screws from the bottom into the base of the shelf.