There are 3 basic measurements for measuring your head:
Around the head
Front to back
Ear to ear over the top
Step 1) Around the head: Start at the hairline in front, follow the hairline behind your ear. then around the back to the bend in your neck, behind your other ear, and around the front to where you began.
Step 2) Front to back: Measure from your hairline in front, back over the top of your head, then down to the roots of your hair on the back of your neck (just below the indent where your skull meets your neck).
Step 3) Ear to ear over the top: Start in the crease where the ear attaches to your head. bring your tape measure straight up directly over the top of your head and then straight down to the same position at your other ear.