@RTB Thanks for reaching out!
@hikermor is spot on with his advice. Depending on your other items and how dirty your down items are, we recommend using a product like Nikwax Down Wash Direct or Granger's Wash + Repel Down 2 in 1. Similarly to your down comforter, if your other items are large (like a sleeping bag) you may need a commercial sized washing machine, which may require a trip to the laundromat. You also want to make sure you use a front loading washing machine.
Personally, I have had great success washing my kids down jackets and my sleeping bag in our front loading machine and tumble drying them on low/medium heat with a couple of dryer balls to help the down loft again.
Hopefully this helps, thanks!