Well, now I think I understand. The problem was, the directions say to pin the backing along the line that has been stitched or marked, but in no place they have said anything on where to mark or stitch that line. So I think it is a flaw in the directions.
As I got a 118" wide muslin, I will make my leaders with a seam on both edges, so the bottom edge will not fray.
Thank you for your answer!