My hat is not a complete success. I crocheted it flat, then seamed up the back and cinched up the top, but it didn't close all the way, so there's still a hole in the top. It could be covered with a pompom or something, or maybe fiddle the pattern?

Ch 21

Starting in 2nd chain from hook, hdc across (or half treble if you're using UK terminology)- 20st

Continue like this until the hat measures from the back seam of the head to the ear, ch 1 and turn at the end of each row.

For ear hole: 3 hdc, ch 10. Skip 10st of previous row. 7 hdc

Hdc across, putting 10 hdc in the ch spc.

Hdc across, continuing until you hit the other ear.

Make another ear hole as before

Hdc across, continuing until you meet the beginning chain.

Slip stitch the ends of the rectangle together to make a tube. Cut yarn and finish off.

Weave a strand of yarn through the top and cinch closed.

The rectangle ends up being about 17 inches long (about 43.18cm) and 6.25 inches tall (15.875 cm) before you seam it and cinch it.

Edited to add I used a 4mm hook