Trim twill tape, available at any craft or sewing store, into four 12-inch pieces. If you have a white or light-colored duvet cover, choose cream or white twill tape to prevent it from showing through.
Fold each piece of twill tape in half, bringing the ends together.
Turn the duvet cover inside-out. Sew a length of twill tape to each corner of the duvet cover, stitching the fold onto the cover and leaving the two ends loose.
Arrange the comforter on top of the duvet cover, with the cover still inside out. Gather one of the corners of the comforter on the end farthest from the duvet opening into a small bunch, and tie the loose ends of the twill tape around the comforter into a secure knot. Repeat on the other bottom corner.
Turn the duvet cover right-side out, with the comforter inside of it. Repeat the process of bunching and tying on the top corners closest to the duvet opening.
Close the duvet comforter. Place the comforter on the bed.