While a down comforter sounds like something that should be cleaned professionally, you might be surprised that you can actually handle washing it yourself. The typical down comforter is filled with little down feathers that are held in place by stitched patterns. Most newer down comforters are heavy duty enough to be washed just like a regular, cotton comforter, but with a little extra care. You do need a large capacity, front-load washing machine, so if you don't have one at home, you should take your comforter to the laundromat.
Down comforters should be washed on a gentle cycle with a small amount of mild laundry detergent, because using too much soap may strip the down. After the wash cycle is finished, run two more rinse cycles to make sure all of the soap is rinsed out. Next, you'll want to dry your comforter on low in an extra capacity dryer. It may take a while on a low heat setting, but you want to make sure that the comforter is dried completely so the feathers don't get moldy. Add a couple of tennis balls or a clean sneaker to help break clumps of feathers up, and take it out a few times during the drying cycle and fluff it by hand. If you want to avoid sitting at a laundromat for a couple of hours waiting for your comforter to dry, you can hang it out to air dry, but you'll still want to put it in the dryer for a little bit to fluff up the feathers. Down comforters shouldn't be washed more than once a year, so use a removable duvet cover to help keep the comforter clean.