The construction of this hat is similar to a traditional crochet hat in that it begins at crown and increases to a desired diameter to achieve the size. The technique of working in the round is taken from the Ipsa CAL by KnitterKnotter.

In this case, once the increases stop, the diameter should be about 6” for a 20-21” diameter head.
A larger or smaller size can be achieved by leaving off/adding increase rows which changes the diameter of crown. Changing hook sizes would also be an option.

Increases are made using the Tunisian Full Stitch. These are made in between established stitches.

Materials:
Interchangeable Tunisian crochet hook: Forward Pass US H/8 (5.0 mm), Return Pass US 7/ (4.5 mm)
2 colors: MC- Forward Pass (background); CC- Return Pass (design)
Rowan Felted Tweed, 50 g, 191 yd/175 m or
Jody Long Alba, 100 g, 362 yd/349 m

Size: Teen/Adult S (for 20-21” head)

Gauge: diameter after Rnd 14 - approx. 6”