Once you've pinned the binding into place, sew the binding to the napkin, about 1/8" from the its left edge.

Begin at your first pin with a few backstitches, sew to the corner. When you come to the corner, leave your needle in the down position, lift the presser foot and rotate napkin 90 degrees. Lower presser foot and sew to next corner.  Repeat for all corners, end at your last pin with a few backstitches.