Buy a specialist down cleaning soap from your local camping store or use a mild non-detergent soap. Your bag will take several days to dry so warm weather with low humidity is the best time to get started.
Fill your bath or laundry tub with enough warm water to cover your sleeping bag and add a small amount of soap, submerge your bag and gently massage the water through it, and leave it to soak for a while. Drain the water then add more fresh water, again, massaging it through the bag.
Repeat this rinsing process until all the soap is gone - you'll need to do this at least three times.
Once the rinse water is clear, drain the bath or tub and carefully press down on the bag until you have squeezed as much water as possible out of the bag. Carefully fold the bag into a bundle and lift it out of the bath or tub – don't let any of the bag hang down as the fragile baffle walls inside the bag that keeps the down in place can be damaged very easily while the bag is wet and heavy.
Dry your sleeping bag flat in a warm place regularly massaging the down to separate it and restore it's loft.