Now that I have mastered making hats from the top down I convert patterns by following this basic formula, primarily for using worsted weight yarn however, it is fairly easy to apply it to any pattern. Determine the number of stitches needed to work the main hat body and follow the guide to that point. There are plenty of tutorials on this method of starting hats.
Abbreviation: Kf&b – Knit into the front and back of the stitch.
Crown: Leave a tail to join the top. Cast on 4 stitches,
Rounds 1 & 2: Kf&b into each stitch (8, 16sts).
Round 3: K1, Kf&b, 24sts.
Round 4 and every alternate round: Knit.
Round 5: K2, Kf&b, 32sts.
Round 7: K3, Kf&b, 40sts, (Preemie 10”).
Round 9: K4, Kf&b, 48sts, (Newborn 0-1yr 12”).
Round 11: K5, Kf&b, 56sts, (Toddler 2-4yrs 14”).
Round 13: K6, Kf&b, 64sts, (Child 5-8yrs 16”).
(At this point you might want to change to a circular needle).
Round 15: K7, Kf&b, 72sts, (Pre-Teen 18”).
Round 17: K8, Kf&b, 80sts, (Teen 20”) (Adult Small).
Round 19: K9, Kf&b, 88sts, (Women’s 22”) (Adult Medium).
Round 21: K10, Kf&b, 96sts, (Men’s 24”) (Adult Large).
I use the decrease bind off method I found explained at Techniques with Theresa to get a loose fit around the head.