To make a tri-cornered hat, cut a circle of felt or heavyweight packing paper for the dome of the hat, cut a larger second circle for the brim, attach the dome to the brim, and fold three portions of the brim up to the top of the dome. Stitch or glue the folds in place, and crumple the felt or paper to create a weathered look.
Use a strip of card stock around your forehead to measure the width for the hat, cutting a circle of enough felt or paper to create a dome around your head and meet at the card stock ring with 1 inch of material to spare. Alternatively, make a one-size hat by cutting a circle 14 inches in diameter. Attach the card stock ring to the inside of the material to form a dome, or wrap masking tape around the outside. Cut the 1 inch of excess into tabs 1/2-inch wide, folding them upward.
Cut a 26-inch circle of material with a hole in the middle to fit the dome. Place glue on the tops of the dome tabs before laying the brim piece over them and allowing them to dry. Fold one-third of the brim against the side of the dome, stitching it in place with embroidery floss. Complete the same procedure for the other two-thirds to create the signature three corners.