The best way to wash a cashmere sweater is by hand. A small amount of laundry detergent and vinegar added to lukewarm water cleans and deodorizes any woolen sweater.
Common wisdom says that cashmere should be dry cleaned, but handwashing is much better for that material. Gently soak the sweater in lukewarm water for 10 minutes. Gently squeeze out water and rinse in fresh lukewarm water until the sweater is free of all detergent. Squeeze out excess water again, and roll the sweater up in a dry white towel to wick more water out of the sweater. Let the sweater air dry.