You should only wash your comforter every 5-7 years at most, and avoid it if you can.
If a full wash is required, machine wash in cold water on a gentle cycle. An oversized commercial or front-loading machine is recommended.
For spills or stains, you can blot the area with a mild cleaning agent (no bleach) on a white cloth.
Tumble dry on a low heat gentle or delicate cycle.
Washing can be avoided entirely by using a duvet cover on your comforter. This will protect it from small stains, as well as dust and body lotions/oils.