The OP is most likely long gone but this might help others.

I do mine in late Spring when the weather gets hot and before the humidity comes up like in summer. Nikwax Down Wash in the bathtub followed by five rinses, gently pressing the water out. Then the bag is spread over the table out back, shaking and turning every hour. Some down will clump in the corners so lightly batting with finger tips will spread that out. As it dries it will fluff. My 20° WM bag takes about 6 hours to dry but I leave it laying across the bed for another day just in case. After the bag is stowed in its large storage bag it always fluffs up way more than before the wash. The process really revives the loft back to near brand new. Just make double sure all the soap is rinsed out and the bag is perfectly dry before storing.

Jackets are far easier and dry much faster too.