I have only tried to make the baby hats that I included with the round ripple blanket but I found this written tutorial on a website called www.slugsontherefrigerator.com about how to design your own hats using any type of yarn and ensuring a good fit every time.
The process is quite logical and should not pose too many problems. The secret seems to be doing a swatch test first and calculating from there.....I can do that.
Have a look at the tutorial for yourself by clicking here.
Will try it myself sometime.....have fun.