Mix 2 teaspoons of bleach in 1/4 cup of water. Use a small paintbrush to paint some of the solution onto a seam of the comforter. Wait one minute to make sure there is no color change.
Place the comforter in your bathtub. Fill a 5-gallon bucket with cool water and 1 1/4 cups of bleach. Pour the solution over the comforter. Repeat until the blanket is submerged in water.
Let the comforter soak for five minutes, then transfer it to the washer. Wash the comforter on a regular cycle with a mild detergent.
Lay the comforter out flat or hang from a line to air-dry.