1) Measure your head circumference with a tape measure around your hairline.
2) Measure from Front to Nape.
3) Measure Ear to Ear across the Forehead.
4) Measure Ear to Ear across the Top.
5) Measure Temple to Temple around the Back of Head.
6) Measure the Nape of your Neck.
Note: Take measurements thrice. Trying to be as accurate as possible. If unsure of a particular measurement it's better to error on the larger side than the smaller.