Then you tape off an 18x21 inch section on your cutting board, making sure your inside diameter is the 18x21, not outside. Your folded fabric will fit inside there with a little extra on the unfolded or raw edges. Cut this off so it fits within your taped square. Two corners will be folded and 2 corners will have the raw edges.
Now line your ruler up parallel to one of the RAW edges, nine inches from the edge and make one long cut.