Place your down coat in the washing machine. Pour in some down-safe detergent.
Place two or three clean tennis balls into a sock. Tie the sock shut and place it in the washing machine. This will prevent the wet feathers in the coat from forming lumps. Additionally, the sock will prevent the fuzz from the tennis balls from rubbing off onto your coat.
Wash the coat in warm water on a gentle cycle. Once the cycle is complete, run a rinse cycle to remove any remaining detergent.
Place the freshly washed coat onto a towel. Press down on the coat with another towel to smooth it out and remove excess moisture.
Put your coat and another sock full of tennis balls in the dryer. Once again, the tennis balls will prove useful in deterring feather clumps. Dry the coat on a gentle cycle.