Down sleeping bags should be washed when dirt and grime begin to interfere with loft. Most manufacturers recommend washing your sleeping bag after 20 nights of use. This is also a good rule for synthetic sleeping bags. In general: if it's dirty, losing loft or smells - wash it!
To wash your sleeping bag, head to your local laundromat and use a large front loading washing machine.
Close zippers. Pull bag out of its stuff sack directly into washer to minimize loft. Bags with waterproof-breathable shells should be turned inside-out. Use warm water and the gentle cycle. Before drying, run an extra spin cycle to remove excess water.
Now is a good time to apply a fresh water repellent treatment. Hang wet bag and spray with ReviveX Spray-On Water Repellent on the bag's exterior. Carefully move the wet sleeping bag into the dryer.
NOTE: the bag will be very water logged and the down plumes will be clumped together - this is normal! The loft will be restored in the dryer.
Dry bag in a large commercial dryer on the permanent press (medium) heat setting. Clean tennis balls may be added to break up down clumps.
NOTE: Home dryers aren't big enough to let sleeping bags tumble freely. The bag needs to be lifted and then free-fall to dry properly. High heat or small home dryers can melt synthetic fabrics. Always use a commercial dryer!
Check progress periodically. Feel the insulation - if there are clumps, the down plumes are still wet. Be patient, it is essential that the down is dried thoroughly. When completely dry, bring your sleeping bag home, unzip and allow to air overnight.