Smooth your baseball shirt across the surface of your ironing board. Set your iron to high heat or its "cotton" setting.
Start at one side of the shirt's bottom hemline, and move the iron gradually across its width. Continue passing over the hemline until the hemline is flat. Turn the shirt over, and repeat for the back hemline.
Mist both hemlines with starch spray. Allow the starch to penetrate the fabric for several minutes.
Iron both hemlines smooth once more. The hemlines should fall straight and flat once you finish.
Repeat these steps after each washing to prevent the shirt from curling during play or casual wear.