Grab a soft measuring tape or a piece of string (like twine or a shoelace) and a ruler or measuring tool
Wrap a soft tape or string around your head to measure about 1/4 to 1/2 inch above your ear and about 3/4 to an 1" inch above your brow line (see image)
Mark the tape or your string, where it intersects, with your finger.
Record your measurement in inches or centimeters (centimeters is often more accurate)
If you're using a string, run it along side the ruler as many times as necessary to get a complete measurement
MEASURE AGAIN!
Always measure more than once until you get the same number twice