The first thing you will want to do is check the care instructions label found on the comforter. There are different fabrics that may require different methods of cleaning. If the label says dry clean only, you will want to follow those instructions. Washing is typically not as gentle as dry cleaning.
For laundering, place the comforter in the washer. Spread it around evenly to prevent the washer from "jumping" during the spin cycle. If it is too bulky to fit in your washer at home, take it to your local laundromat and put it in a front loading commercial size washer. If you prefer, you could fill your bathtub and wash your comforter by hand. After draining the wash water. Rinse a couple times to be sure all the soap in rinsed out.
Select the water temperature to warm/cold and the gentle cycle. Because a comforter is fluffy, use less soap than you would for a regular load of clothes, about 1/3 of what you normally use. Choose a detergent with low suds as excess suds cuts down on effective cleaning power. DO NOT BLEACH or use detergent with bleach in it. Avoid fabric softeners as well as it can damage the fill material. To help in the washing process, you can put clean, white tennis balls or down fluffer rings in with the comforter when you wash it.
If it's sunny out, you can lay your comforter out flat or hang it up to dry. Fluff it and flip it over every half hour or so. It takes a long time to dry, possibly up to 12 hours. If you see clumps when you hold it up to the sun, it's not completely dry. If you dry in the dryer, set it on air dry. Avoid dryer sheets as these too may damage the fill. Again, it takes a long time. You can use the tennis balls or down fluffer rings in the dryer to help fluff it while drying.
Be sure your comforter is completely dry before you store it. Otherwise it will mold or mildew.