Measure the length from the corner of the wall to the point at which you want the shelf to extend.
Draw a circle onto a piece of cardboard that has a diameter that’s double the wall measurement. Use a compass for assistance.
Draw four equal segments onto the cardboard circle, using a ruler to form a straight line.
Cut out the circle with a utility knife. Cut along the two lines that divide the circle into the four segments. This creates four equal pieces, any of which you can use for the shelf template.