Jeremy has a book of Wizard Craft that is full of projects closely resembling a lot of items and words and stories from Harry Potter, but it's not "officially" Harry Potter. Owen has been looking through at all the projects recently and announced to me "The first thing I want is a wizard hat. Mom, you know how to sew so that can be your job."
We measured around Owen's head. Multiplied that measurement by 4 then divided that number by 6.28. That length was the length of each of the two straight sides of the hat pattern. (see the picture) I measured them on the corner of the newspaper so they would be at a 90 degree angle from each other. Then draw a curved line between them. (The most precise method is to use a piece of string the right length anchor it at the point of the pattern and swing the string from one side to the other marking a curved line on the pattern.)
We pinned the pattern on the fabric and cut it out adding a half inch seam allowance. Then we cut out a piece of interfacing the exact size of the pattern. We ironed that on the wrong side according to the interfacing directions. With right sides facing we sewed the two straight sides of the hat together. Then turned it right-side out and sewed under the brim.