Cut the backing fabric to the duvet dimensions, adding 3" to the length and width. If necessary, piece together fabric to achieve the desired dimensions.
Cut fabric panels to the duvet length, plus 3" (a on "Duvet Dimensions" chart). Align the fabric widths right sides up on a flat surface, matching the motif repeat if necessary (5). Right sides together, stitch the panel long edges. Zigzag-finish and press open the seams. Trim any excess fabric if necessary.
Fold the fabric and backing lower edge 1/2" to the wrong side and press.
Cut ribbon lengths to the duvet length, plus 3" (a on "Duvet Dimensions" chart). Cut as many ribbon lengths as there are panel seams. Fuse the seam tape to the ribbon wrong side following the manufacturer's instructions. Fuse the ribbon to the fabric right side along the seamlines. Topstitch the ribbon in place.
Cut twenty 8"lengths of ribbon. On the fabric right side along the folded lower edge, pin 10 ribbon short ends equidistant from each other (6). Topstitch the ribbons in place. Fold the fabric lower edge another 1/2" toward the wrong side and press. Topstitch close to the second fold. Knot the ribbon ends and, if desired, apply seam sealant to the raw edges to prevent fraying.
Repeat to stitch the remaining ribbons to the backing right side lower edge.
Right sides together and aligning all corners and ribbons, generously pin the backing to the duvet fabric. Stitch, leaving the lower edge open (7). Turn the duvet cover right side out. Insert your comforter into the duvet cover, matching the corners, and knot the ribbons to close the lower edge