Use a rigid ruler or measuring tape
Measure when fully erect for consistency
Place the ruler on the top side of the penis
Press the ruler gently against the pubic bone
Measure from the pubic bone to the tip of the glans
Do not include the foreskin beyond the tip
For girth, wrap a soft measuring tape around the thickest part
Keep the tape snug but not tight
Record the measurement in inches or centimeters
Repeat a few times for accuracy