Use a flexible measuring tape or a ruler
Measure when fully erect for length
Measure from the top side of the penis at the pubic bone to the tip
Press the ruler or tape firmly into the fat pad at the base until it meets the pubic bone
Do not measure from the visible skin surface only
For girth, wrap the measuring tape around the thickest part of the shaft
If using a ruler for girth, use a string first, then measure the string
Measure in a private, comfortable setting
Repeat the measurement a few times and use the same method each time
Record both length and girth separately
If you want an accurate comparison, use the same position and erection level each time