The following information and pictures have been provided by the site that helped me the most, that is of course Penis-Health.com. At Penis-Health they also have larger pictures and better quality pictures as well as videos tutorials on how to measure your penis. This can help if the pictures are not clear enough below:
Measuring The Flaccid Length:
The flaccid length of the penis can be dictated by many things, including the temperature. This might mean that you have contradicting results depending on the temperature of the situation that you are in. Flaccid gains are the first gains that come with a penis enlargement program, to keep results accurate we have a certain standardized way of measuring the flaccid penis. To measure accurately it is important that you measure from the side of the penis. Stand up straight and extend your flaccid penis in front of you (it important NOT to stretch it), your penis should now be parallel with the floor. Place the ruler next to your penis so it is slightly pressing against the pubic bone. The ruler and your penis should be parallel with each other (as shown in the video).
Measuring The Erect Length:
The erect length is what most members are interested in. Again it is best to measure from the side of the penis. This method is the standardized method that researchers use to measure the "average penis size" which is incidentally between 6" - 6.5" erect length. Firstly make sure you have a 100% erection where you would consider it the "most erect". Hold your penis below the head and place the ruler next to it with your other hand. The ruler should be slightly pressing against your pubic bone. Measure to the nearest millimetre and it is important that you do not EXAGGERATE the measurement, if anything it is recommended that you under exaggerate. Exaggeration on the measurement could lead to a lack of motivation in the future. Make sure you do the right one!
Measuring The Flaccid Girth:
The girth can be extremely awkward to measure because many people have different size girth measurements depending on which part of the penis that you measure. The standardized way is to measure in the centre of the penis shaft. When flaccid hold your penis out in front of you and wrap a tape measure around your penis. Do not pull the tape measure too hard, pull it to a reasonable level and remember this LEVEL of pull for the next time you measure. The average flaccid length of girth is between 3"-4". In our photographs the model as a large flaccid length and girth it is likely that yours will be smaller.
Measuring The Erect GIrth:
Make sure your penis is 100% erect and then proceed with the measuring. Hold your penis out in front of you so it is parallel with the floor. With your other hand place the tape measure around the penis making sure not to pull too tight. The average erect girth is 5" - 5.5" according to recent surveys conducted.