In order to compare to the values described here, you should measure yourself properly. That said, there's still some disagreement on what the standard should be, as some datasets use different measurement methods than others. Pay attention to the methods used by the dataset you select, as described in the dataset list.
Erect length should be measured when you're fully hard with a ruler (or some other similar measuring tool) along the top of your member comfortably pressed against the pubic bone (Bone-pressed). Do not press the ruler below the pubic bone. For datasets that use Non Bone-pressed measurements, simply hold the ruler in the same position, but don't push it against the fat pad in front of the pubic bone.
If you have a strong/moderate curve, you can either gently straighten the curve, use a piece of string that goes along the top following the curve, or disregard the curve completely, which method you choose to use will inform you about different comparative aspects. You should note that many of the studies exclude people with penile curvature.
Stretched Length should be measured similarly, but with the penis fully flaccid and the head pulled outward as far as comfortably possible without causing pain (yes I'm sure you can stretch it farther, no doing so will not yield more accurate percentile results, and no I am not responsible for you injuring yourself). The stretched length is well correlated to the erect length, however studies have more potential to under-stretch the length since it is being stretched by another person trying not to cause discomfort, almost always leading to a slightly lower stretched average compared to the corresponding erect average. Additionally, due to its imperfect correlation, an individual's stretched length can be longer or shorter than their erect length.
Flaccid Length should be measured similarly, but with the penis completely flaccid and unaltered from its normal contracted state (and before being affected by exposure to cold temperatures), except to hold it perpendicular to the body.
Girth (circumference) can be measured using a tailor's tape wrapped around the penis. If you don't have any tape, you can make your own by drawing the lines of a ruler onto a strip of paper. Studies often measure at different points along the shaft, but the resulting averages are usually very similar, so there is no consensus if girth should be measured along the base, the middle, or by an average of the two, so feel free to use any of those measurements, but do not measure it along the glans/head. It is measured in the fully erect state for erect girth, and the unaltered flaccid state for flaccid girth.
To accurately get percentile estimates you should use an honest measurement as if you were a researcher measuring another person, in other words don't try your hardest to get the maximal dimensions.