Put comforter in tub

Fill tub with cold water

Add laundry detergent about halfway through

Swirl it around with your hands (make sure you wear gloves, laundry detergent usually has bleach in it) and make sure at some point every part of the blanket goes under water. My tub is larger than most tubs (lip to lip is just under 6' long instead of 5' long) and it still can't completely fit in at once.

Drain tub

Repeat previous steps with cold water again but no soap this time to rinse it out

Let it dry out for a couple days

Enjoy clean comforter