Making a cover for your rainbow comforter set is a great way to add years to its life. You can wash the lid instead of washing the blanket, keeping feathers or other blankets filling fluffy and soft. Probably the cheapest way to make a duvet cover is to use the flat sheets the same size as the duvet. These instructions are for a queen-size scar that is 92 inches square. You can easily change directions to make other duvet covers sizes.
Iron sheets to remove creases, which interfere precisely with measurement and cutting. Place a rainbow comforter on the floor or a large workspace and mark a rectangle 93 inches wide and 95 inches long with a fabric marker. Avoid the hems of the sheet when marking the rectangle. Cut out the rectangle. Repeat with the second sheet. Fold over a shorter side of a rectangle one inch. The right side of the sheet is the one you see when the duvet cover is on the bed. Sew the hem using a sewing machine. Repeat to make a hem along a short side of the second rectangle you have cut.
Turn a piece of fabric so that the right side is facing up. Pin a 69-inch long strip of snaps along the hem you just made. The strip should be aligned on the short side of the comforter set so that you have about 12 inches of fabric on either side of the strip. Do the same with another strip of snaps along the hem of other piece of fabric. Sew strips of snaps to fabric pieces using a sewing machine with a zipper foot. The zipper foot allows you to sew along the belt without bumping into snaps.