Fill the sink deeply enough to dip the crown of the hat in the water. The water should range from warm to hot.
Dip the bowl of the hat into the water and get it thoroughly wet. Do not dip in the bill of the hat, as it will become soft and shapeless if wet.
Remove the hat from the sink and wear it while still wet. As the hat dries, it will shrink slightly, conforming to the shape of your head.