Here's a link from one of my favorite down manufacturers (made in Seattle!) with detailed directions for caring for your down gear and garments: https://featheredfriends.com/pages/washing-care
A lot of folks know about using dryer balls to help prevent clumping. Another good trick is to turn the garment inside out, so that the water can drain through the lining, which is generally more thin and permeable than the outer shell. Before moving the item to the dryer, gently squeeze it to remove as much water as possible (remember down gets really heavy when it's wet!). It's also possible to clean down-filled items by tub-soaking, but never use a washing machine with an agitator as it could snag and tear your garment.
Two tried-and-true down cleaners are Nikwax and Revivex. I've found both pretty effective. I have down-filled items that have been in regular use for a decade or more, and I think washing once a year has helped keep them in rotation. It's good to remove dirt and oils from the feathers to keep the insulation lofty.