Melissa from Blank Slate Patterns and Melly Sews shows how to draft and sew a basic hat pattern. She includes a free template for making the crown. The template has sizes ranging from 19 1/2 to 24 1/2?, which covers babies through adults. Her tutorial is part of a month-long series on sewing menswear, though this would also work to make hats for women and girls. Go to Melly Sews for the tutorial and to download the crown template.
[photo from Melly Sews]