There are many ways to shrink a baseball cap, including placing it into a pot of very hot water, washing it in a laundry machine at the hottest possible setting or using a spray bottle to moisten the cap and shrink it to the right size. It is important to never place a baseball cap in a pot of boiling water. The water must be very hot, however.
To shrink a baseball cap in hot water, a bucket, pot or kitchen sink must be filled with very hot water. The water can't be boiling though - this is very important. The hat must then be completely submerged in the hot water. Once the hat is soaked, it should then be taken out and placed on the person's head while it is still wet. The idea here is that as the cap dries, it will shrink to fit the size of the person's head.
The brim of the baseball cap can also be adjusted as well, Using a spray bottle, the brim should be sprayed plenty of times but not soaked. A baseball or softball should then be placed underneath the brim, and then rubber bands should be wrapped around the ball and the brim of the hat to ensure that the brim reshapes to the curvature of the ball. This could take some time, usually a day or two. This process can also be repeated as many times as needed.