What's the best way to clean a down quilt?

Put it in the washing machine on the delicate cycle and spin only for two minutes. Use a mild soap like Dr. Bronner’s Sal Soap.

Then dry on low heat in the dryer with several tennis balls to help fluff up the down.

We are not talking about an antique hand-quilted quilt, are we? If so, then forget my advice.

No, it’s not valuable in any way except that it’s very cozy and comforting.
Have you done this? I’ll have to find a friend with tennis balls. Would Ivory Snow work?

@gailcalled has it right. Wash it on gentle with a mild detergent and rinse it. I have done this and it works.

I have also dried it without the use of tennis balls, but I’ve heard of this technique. You want the lowest temperature setting on the dryer.

One question: is this a box stitch design or some other? In other words, are the down feathers divided into separate boxes in the quilt? If so, this is good. It will reduce shifting of the feathers during washing. If not, then be prepared for the feathers to work themselves to the edges of the quilt.

Edit to add: use a duvet cover to reduce the necessity of washing the quilt itself.

Ivory Snow would be fine. If you don’t have tennis balls. you can remove the laces from a clean sneaker and throw that in the dryer.

I put them in the washing machine at regular cycle. Then dry on medium with sneakers to unclump the feathers. And as @gailcalled suggest a doona cover (as they are called here) helps to keep them clean.

I don’t know if I would dry it in a dryer. I have a down jacket, a couple of down pillows and a duvet. I wash them all in the machine, using a wool wash liquid, but I always hang them to dry, giving them a bit of a shake and fluff a few times during the time they are hanging.

It’s fine to wash on gentle cycle, but I don’t recommend using a dryer. My old boss at the drycleaners would not approve. He told me that tumble dryers will loosen and weaken the stitching of the quilt, and that typically a not-new quilt will fall apart and loose feathers into the dryer, causing it to need rebuilding. Then he said, “ask me how I know…”
Hang the quilt to dry. Turn it frequently. Don’t fold it if you can avoid doing so (you probably will have to fold it at first, wet down quilts are heavy and will tend to fall off the line).
Turn the quilt end-for-end every few hours to keep the feathers from clumping. It can take up to 24 hours to dry a king-size down quilt.

What do you do about drying if you live in an apartment and don’t have a line? Sounds like my best bet would be to take it to the cleaners.
Thanks for all your answers.