Ready-made duvet covers can be pricey, and you may not be able to find one with just the right color or pattern to complement your sheets. Make your own perfect match with two flat sheets.
Martha Stewart Living, October 2002
Use a pair the same size as the comforter. Place them right sides together, and sew around the perimeter with a 1/2-inch seam allowance, leaving an opening along the bottom that extends to 24 inches from each corner. Turn right side out.
To fasten cover shut once the comforter is inside, sew twill-tape ties, buttons and buttonholes, or Velcro tape along the two inside edges of the opening.