Use a soft measuring tape or a ruler
Measure when the penis is fully erect
For length, place the ruler at the pubic bone on the top side
Press the ruler gently into the fat pad until it reaches the bone
Measure to the tip of the glans, not including the foreskin
For girth, wrap the measuring tape around the thickest part of the shaft
If using a ruler for girth, measure the circumference with a string first, then compare it to a ruler
Measure in a private, relaxed setting
Repeat the measurement a few times and use the average
Record the result in the same units each time