I think we should bring hats back. This is a hat from history that I like. They are a good beginner's sewing project because they're small, they're all straight lines, and it rewards precision.
The short version is this SVG and these steps:
- Cut two triangles in contrasting fabric. They should be isoscles with a base equal to the circumference of your head minus a couple cm and a band equal to the length from the base of your ear to the top of your head plus a couple cm.
- Put the triangles opposite way around and sew the slanty sides together.
- Fold each triangle over onto the other one (making sure that you don't turn it into a möbius strip) and sew the other slanty sides together. Now you should have a rectangle with a diagonal.
- Seam along the top.
- Sew a neat hem.
I wrote longer instructions on this Nottinghack wiki page.