Fold the canvas in half lengthwise. Starting at the hemmed edge, mark six inches from the edge on the folded side and 20 inches from the edge on the hemmed side. Use one of your stakes as a guide and draw a diagonal line from point to point. From that bottom point, on the hemmed edge mark over 10 inches and mark 38 inches from the line on the folded edge, draw a diagonal line connecting your points. Repeat in reverse, marking 10 inches from the line on the fold and 38 inches along the hem, draw a diagonal line.