Place the 1-by-12-by-28-inch board with the 12-by-28-inch side flat against a work surface.
Draw a curve on the corners of one 28-inch side of the board with a pencil and flexible curve ruler. The curve is decorative and can be drawn to your aesthetic preference.
Saw the curves off the board using a band saw. Push the board through the saw steadily without forcing it as you guide it along the pencil lines to cut each curve off.
Sand the curves and other surfaces of the board with 120-grit sandpaper. Wipe off the board with a tack cloth to remove the dust left from sanding.
Apply an even coat of wood finish using a paintbrush. Allow the finish to dry for 24 hours.
Attach one shelf bracket on each end of the bottom 12-by-28-inch side of the board. The curved corners are the front of the board. Orient the back of the brackets along the back 28-inch long side of the board. Locate one shelf bracket 6 inches in from each end. This will allow you to mount the shelf to wall studs, which are spaced at a standard 16 inches on center in walls.
Drive the provided screws through the holes in the brackets into the bottom of the board with a screw gun to fasten them in place.