Measure the circumference of your (or the wearer's) head and then add 1/2" to that measurement. Cut a piece of felt with the center circle opening the circumference measurement. Make the brim 4" to 6" wide.
Cut a rectangle of fabric that is the head circumference in width by your desired hat height (about 10” to 12” is good). Fold the fabric in half. On the folded rectangular piece, cut an arc, starting at the bottom fold arcing up towards the top outer corner. You should now have something that looks like a half circle.