I’m happy to see so many Boyfriend Hats here! :) :)
This hat is a skull cap knit in the round in 3x2 ribbing. One size ideally fits a 23.5” head circumference; however, this pattern is made in super stretchy ribbing, so the hat can fit slightly wider or smaller heads as well (or you could cast on more or less stitches in multiples of 10 and still follow the pattern as written). Also, this hat was designed to cover just the tops of ears. If you want a hat that fully covers the ears, knit an extra inch (in ribbed pattern) before starting the decreases.
The yardage is given as 2 full skeins; however, the hat actually takes over 1 skein but less than 2 of the Mission Falls wool, but remember: you might need more or less if you use a different yarn or get a different gauge.
Errata: Row 5 of the decreases should read: Row 5: ^K3, P1, P2tog; repeat from ^ until end of row (50 sts remain). The pattern has been fixed as of 2007-10-27. (Thanks, archaeoknits!)