Sorry it's unclear.
"Picture3: Fold backing fabric lining up cut edge with corner of pieced top on the fold. Place a pin in the middle. Now stitch from folded corner to the pin. Clip corner."
The top is the sliced off edge you see in picture 2. The white line is the stitching line. The pin will keep you from stitching too far.
If it still us unclear, this weekend I will get my DH to help me take a few more pictures or a short video. I noticed this weekend that our camera will do like a 15 second video.