To wash a baseball cap, start with spot cleaning by spraying any stains with a stain remover. Let the stain remover sit for a few minutes and then wash it off. Next, fill a sink with cool water and laundry detergent and submerge your baseball cap in it. Use a cloth to rub the soapy water into your hat, and then rinse it with cold water. Finally, lay your hat on a round object to air-dry so that it holds its shape. If you want to learn how to wash your baseball cap in the dishwasher, keep reading!