To measure the length of the penis, you will need a wooden or plastic ruler. You will need a full hard erection.
Press the ruler into the pubic bone at the top and base of the penis next to the body. The actual length is from the base to the tip of the penis.
To measure girth, you will measure either thickness, which is also width, or the distance around which is the circumference.
To measure width, place a ruler across the widest part of the penis and measure from one side to the other.
To measure the circumference, use a cloth tape measure or a string. These cannot stretch as it would give a false reading. So, no yarn. For string, wrap around the thickest part of the shaft and make a line on both ends of the string where it overlaps. Then measure on a ruler from line to line. Using a cloth tape measure, you will wrap it around the thickest part and line up the zero line to where it overlaps and that is the circumference.
Or use your width measurement and divide it by 3.14 to get the circumference.