A duvet is a down comforter, ideally filled with eiderdown. Because eiderdown is such a good insulator, an eiderdown duvet is flatter than a down comforter made with less desirable down. Duvets are generally used with a removable washable cover, which keeps the comforter clean. In the U.S., the word "duvet" often refers to the removable cover as well as the comforter itself, leading to a little confusion. If you have a down comforter you want to protect, you can make an attractive duvet cover out of bed sheets. The cover is essentially a giant pillowcase that slides over the comforter and is tied in place with ribbons.
Measure your comforter; add two inches to the width and four inches to the length. Trim two sheets to these measurements.
Pin the sheets together with right sides facing each other, wrong sides out. Sew a 1/2-inch seam on three sides, leaving one of the shorter sides open. Trim the corners, turn the duvet cover right side out and press the seams and corners.
Fold over 1/4 inch of the raw edges along the open end. Press and then fold over another 2 inches and pin in place. Sew this hem down all around the opening so that the cover resembles a giant pillowcase with a 2-inch hem. Press the hem.
Cut the ribbon into 8 pieces. Sew one piece to each of the upper corners of the comforter and one piece inside each corner of the closed end of the duvet cover.
Sew the remaining four pieces of ribbon on the open end of the duvet cover, 2 on each side, 1/4 of the way along the hem from the open corners.
Turn the duvet cover inside out. Tie the ribbons in the corners of the duvet to the ribbons on the comforter. Turn the duvet cover out again, with the comforter inside. Spread the comforter out evenly inside the cover. Finally, tie the open end closed with the ribbons you sewn to the hem. Place on your bed with the closed end at the head of the bed and the open end at the foot of the bed.
If you prefer to buy yardage instead of sheets, you will have to piece the yardage together to make each side of the cover wide enough to cover the comforter. Center one strip of fabric and then sew additional yardage on each side to make the fabric wide enough. This looks better than having a seam right up the centre of the duvet cover. If you prefer the look, use buttons and buttonholes to close the open end of the cover.