It is important to us that you get your head measurements right.. You would need a tape rule and a mirror to do this.
Circumference: Circular measurement of your head. Measure from ear to ear and back to the same ear you started from. Picture below
Front to nape: This is the back of the neck. Measure from your hairline, along the middle of your head and to the back of your neck to your head
Ear to ear across the forehead: Measure from one ear to the other along your forehead
Ear to ear over top: Measure from one ear to the other across your head.
Temple to temple round back: Place one end of the tape on your temple then wrap the tape around the back of your head all the way to your temple
Nape of the neck: Place one end of the tape behind your ear and follow the nape of your neck around to your other ear. A good place to start and finish is the small crease where a pair of glasses rests.
Here is a picture of how to get it done