The Rectangly Hat involves a technique called double-knitting – basically knitting two layers at the same time. You end up with a double-thickness hat, so one that will be lovely and warm and toasty. The hat is also reversible. If you’ve never tried double knitting before, give it a go – this pattern starts off gently with some easy non-patterned rounds to get you started before moving into the rectangular patterns.
I used a DK weight yarn, but a light worsted weight should work as well.
There are lots of descriptions/instructions and a photo-tutorial to help you with decreasing for the crown.