STEP 10: Using a ruler, draw a straight line across the seamed corners with tailor’s chalk or a pen to create a triangle. Your line should measure 4" across. Pin on both sides of the line. Make sure that your lines are parallel with one another and that the opening forms another straight parallel line. Sew along the chalk lines, making sure to lock stitches. Trim off the excess fabric (snip the corner).
Photo Courtesy John Wiley & Sons, Inc.