Always check the cleaning instructions on the label first.
Preferably wash your down sleeping bag by hand. This avoids unnecessary risk. You can leave it soaking for a while so the detergent penetrates fully. Rinse thoroughly, several times if necessary.
You may be able to machine-wash it in cold water or at a low temperature. In that case, preferably wash the sleeping bag on its own. Choose a delicate cycle. You can add tennis balls to the drum to keep working the bag and maintain an even distribution of fill.
There are specialist detergents for washing down, which I recommend. They preserve loft and rinse out easily. If using your usual detergent, I think it is better to use less than the normal dose.
Here are the detergents we offer at RayonRando.com: