Creating a baseball cap may require some practice to get right. Sew together six triangles of a sturdy fabric to form the dome. Seal the connection at the top with a snap button. Make the brim using cardboard and fabric, then sew that to the dome.
The key to success in creating your own baseball cap is proper measurement. Make sure to measure the circumference and height of your head carefully. One way to make this a little easier is to use an existing cap, if you have one, as a guide.
When you are cutting the triangles for the dome, their combined bases should be equal to the circumference of your head plus a few extra inches for overlap. Their heights should be about the distance from your ears to the top of your head plus about an extra 1/4 inch or so to form a hem.
The brim of your hat should be cut to match the contour of your forehead. Use an existing cap as a guide to help with this. Choose a cardboard or plastic that is soft enough to sew through but sturdy enough to hold a shape.
Finally, do not forget to cut out ventilation holes in the dome of the cap. Embroider the edges of these to prevent fraying.