• Use a soft brush such as an unused soft-bristled toothbrush or a hat brush specially designed for felt hats.
• Start on one side of the hat and work your way around with short, gentle counter-clockwise strokes along the direction of the fabric's nap to maintain the material's integrity while removing dust, dirt, and lint.
• For any noticeably stained area of the hat, you can carefully dab it and spot clean with a slightly damp cloth or sponge (such as a makeup sponge). Make sure the cloth is not too wet.
• To absorb and remove oily stains caused by hair products, makeup, lotions, or body oils, sprinkle the stained area with cornstarch and leave for at least a couple of hours before brushing away.
• Repeat any or all of these steps as necessary.

Cleaning the Entire Hat

• Fill a bowl with cold water — never hot water as that will very likely ruin the hat.
• Add a small amount of mild, non-bleach laundry detergent (about a teaspoon) such as Woolite until it's dissolved.
• Before you dunk the entire hat in the cool water, spot test it first by soaking a small, unnoticeable part of it for a few minutes to make sure the color doesn't bleed.
• Gently brush away any stains with a soft-bristled brush such as a toothbrush or felt hat brush and allow it to set for at least 15 minutes before hand washing.  
• Once that's done, submerge the entire hat in the bowl for 30 minutes.
• Remove the hat from the soapy water and rinse in cold water to remove any leftover detergent. 
• Place the hat on a clean towel, on a flat surface and gently pat it down.
• Air dry the hat in a clean spot away from direct sunlight.

Wavy / Warped Brim

• Designate a clean flat surface that can withstand heat.
• Use your iron on steamer setting (make sure it's pushing out a lot of steam) and lightly brush the hot air around the brim either on the specific spot you're trying to flatten, or the whole way around.
• Immediately after steaming — and while your hat is still hot and soft, take it over to the designated surface and lay it flat.
• Push down on the brim to make it even flatter! (Tip: use a few heavy books to get that brim really flat.)
• Count down 5 minutes and voilà! Your brim should be stiff again and back in shape!
• Repeat the process as needed until the desired shape is achieved.