Inspect the comforter for weak seams or fabric that may give out during washing. Reinforce any weak spots with stitching or place the entire comforter in a mesh laundry bag if the fabric is weakening in several spots.
Wash the comforter on the gentle cycle with warm water. Use a mild laundry detergent in the amount specified on the package. Laundromats have large machines available for washing bedding if your home machine is too small for the comforter.
Run the rinse cycle two to three times to ensure all the detergent is removed from the comforter.
Machine dry the comforter on a low-heat setting. Place two clean tennis balls in the dryer. These bounce around and add loft to the down while speeding drying time. Large down comforters may take up to six hours to dry completely.
Split one seam in the comforter to access the down inside. Keep the down stored in the comforter cover until you're ready to repurpose it.
Fill a new comforter cover with the clean down, or use the down to stuff pillows or pet beds. Down also makes a suitable stuffing for sleeping bags.