Remove the duvet cover from the comforter.
Tumble dry on low heat to remove wrinkles if the cover doesn't require washing. Lay it flat and smooth wrinkles with the palm of your hand. Iron according to manufacturer's instructions if necessary.
Wash the duvet cover with mild detergent on your washer's delicate cycle if it is dirty and wrinkled, or if the wrinkles don't smooth during the tumble-dry process. Use a commercial-sized washing machine for thick covers that don't fit easily in your home washer. Shake the cover thoroughly before drying to release wrinkles.
Tumble dry on low heat, lay flat and smooth the wrinkles immediately. Alternatively, hang the cover on a clothesline for 24 hours.
Iron according to the instructions on the tag if necessary. Allow the cover to fully cool. Insert the comforter into the duvet cover and lay it flat on the bed.