There are three different methods for applying a patch to a baseball cap. The methods are ironing the patch, gluing the patch and sewing the patch. Depending on the patch and how you are going to use the cap, any of these methods would work fine. Some patches are iron-on and you can iron them onto the cap. If the patch is not iron-on, you will need to either glue it or sew it onto the baseball cap.
Using the iron, attach an iron-on patch to the cap. Place the patch on the desired location (holding it in place with needles works best), put a thin towel over the top, and apply a warm iron with even pressure to the patch. Allow it to cool, remove the needles and the patch will be good to go.
Glue the patch onto the cap by applying fabric glue to the back of the patch and sticking the patch onto the desired location of the hat.
Sew the patch onto the cap, starting with a needle and the appropriate color of thread on one side of the patch, pushing the needle through the cap and patch and around the edge of the patch. Work your way around the edge of the patch until the entire patch is sewn onto the cap. When you reach the starting point, simply tie the thread in a knot on the back of the patch and you are done.