Use a large front loading washing machine with down specific wash.
Don't use a top loading washer! It will wrap your bag around the center column and may cause permanent damage.
Dry the bag in a LARGE dryer on minimal heat (so you don't melt the nylon shell) with a few tennis balls to help break up the wet down clumps.
Once your bag is out of the dryer, shake the bag to redistribute the down within the baffles as needed so that you don't have cold spots.
Marmot will wash sleeping bag for a fee also. Go to their website www.marmot.com