Take some fingering-weight/sock yarn and cast on 104 sts on size 4 or 5 needles, working in the round.
K2 P2 for an inch, followed by stockinette st for 6 or so inches. Flip it inside out and finish with 3 needle cast off.
Weave in ends and place securely on cold head. Fits a large 3 year-old head, or normal-sized dome on an older child.