Before you begin to measure your head, make sure your hair is all pinned up and lying flat (with or without a wig cap). Prepare your hair similar to the same way you would have it right before applying the wig. This will eliminate bulges that distort head contours. Use a cloth tape measure to get an accurate reading.
 

Measure Around the Head: 
Starting in the front at your natural hairline, follow along your hairline, just behind your ears. Continue to bend of neck and where the nap of your neck meets, over other ear, to starting point

Measure Front to Back:
Measure from the front of your natural hairline back over the crown of your head to where your head bends and meets the nap of your neck.  If you have little or no hair, lay your index, third, and ring finger flat against your forehead just about at the brow bone. At the top of the three fingers would be a good straight point. Measure until about an inch below the small bone at the bottom of the skull.

Measure Ear to Ear: (Full Lace Frontal Closure Measurement)
Start in front of one ear, lay the end of the tape about a half inch above your little crease (where glasses rest) at the top of your ear between the ear and the head. Bringing your tape measure over the crown of your head to the front of your other ear in the same location as the first ear.

How to Measure for a Wig or Full Lace Frontal Closure