There are several ways to shrink a fitted hat, but heat is the key factor in the process. Most hats are a cotton blend, and cotton is fairly easy to shrink. It's actually difficult to keep a hat from shrinking.
The one-step method for shrinking a fitted hat involves putting it in the dishwasher. Since the dishwasher cleans dirty dishes, the water gets hot. Plus, it has the steam dry option which creates a lot of heat, as well. Simply place the hat on the top rack and run through a wash cycle without any soap. Since the dishwasher washes and dries at high heat, this method is only recommended for those who need maximum shrinkage.
Other methods for shrinking a fitted cap include letting it soak in a bucket or tub of hot water for 30 minutes and then drying in the tumble dryer. Spraying it with hot water from a spray bottle and letting it dry in the sun also works. For those people who need a medium amount of shrinkage, soaking and drying in the dryer is the best option. For those people needing a small amount of shrinkage to get a snug fit, the spray bottle and drying in the sun is the best option.