Use a rigid ruler or measuring tape
Measure when the penis is fully erect
Place the ruler on the top side of the penis
Press the ruler gently into the pubic bone
Measure from the pubic bone to the tip of the glans
Do not include the foreskin in the length measurement
For girth, wrap a measuring tape around the thickest part of the shaft
If using a string for girth, mark it and then measure the string with a ruler
Measure more than once for consistency
Record the measurement in the same units each time