Once upon a time, I had a baseball cap. And I wore it out. I took it apart with a seam ripper, rescued the plastic from inside the bill, and used the pieces as a pattern to make myself a new cap in a more feminine fabric. That one also wore out, eventually. I rescued the bill again, and threw the rest away. When I got to sewing another one, I measured my head, drew up a pattern from measurements and memory, and made a new cap with interesting fabric from my modest fabric stash. I had one of my husband's caps for reference, for the tricky parts like how to attach the sweatband. (Even then, I sewed it on wrong the first time.)
The lowly baseball cap is both simple and intricate in construction at the same time; I find sewing my own to be moderately challenging. The main body of the hat is six "fat triangles" of equal size, sewn together. The seams are covered with a seam tape, which is topstitched. At the top, a button covered with fabric is attached where the tips of the triangles meet. At the back, a semicircle is cut out from the bottom of one triangle, and two straps are attached; in my hats I connected these with a buckle, or just by tying them together. The sweatband is a folded piece of fabric that is stitched to the bottom edge of the hat, and then folded up inside. The bill is a piece of plastic or cardboard between two sewn layers of fabric, then several lines of stitching go through all the layers. The bill is attached to the rest of the hat by stitching. Precision at every step pays off.