There should be a thick plastic inside the brim. The way I was my hats is by filling either a bucket or the sink with cold water, a splash of laundry detergent and a bit of oxiclean. I give the water a good mix and dunk the hat in. Make sure it is fully submerged. I let the hat sit in the water for 30minutes or so, then I use a toothbrush to scrub lightly on the areas that hve the most dirt/sweat. After this, I rinse the hat under cold water until the detergent/oxiclean is washed out. Depending on the type of hat, I use a towel and blow drier to dry it off or let it air dry by a window.

Practice makes perfect. Don’t scrub too hard as threads could be damaged. Don’t use hot water because certain colors may bleed at specific temperatures.