There are two standard way to measure bone pressed, from the side or from the top.
From the side has the advantage of being a lot easier to get a stable reading without pressing like fuck into the ligs. From the top will measure shorter (probably) but it has the advantage that the bone across the top is fairly straight whereas down the side curves in as you go lower.
To measure either: take a ruler push hard into the bone above or to the side of the penis and measure the distance to the glans. Its preferable to have a ruler where zero is the end of the ruler (for me at least its essential as my little head won’t carry information like having to add X/16 inch to get the true value).
The point of the Bone pressed measurement is to create a solid measurement so you must be at your highest erection level. A small variantion from your highest might result in differing measurements so that you aren’t actually measuring gains but instead are measuring how good an erection you have.
An alternative is to measure bone pressed flaccid, where you push to the bone again and this time pull your flaccid penis out as far as possible. Some people find this produces a more easily repeatable measurement (personally I find it impossible to use).
So you now have your bone pressed length measurements (BPEL) or bone pressed flaccid stretch (BPFSL).
Do the erect measurement again but this time don’t push to the bone simply rest the end of the ruler at the base of the penis. This is the non bone pressed erect length (NBPEL).
Also take a tailors tape or a piece of string and measure the girth of the penis at the mid point (and if you are a measurement fetishist at the base, below the glans and the glans themselves).
You now have a complete set of measurements. Its often worth measuring each type three times and taking an average. Also you need to leave at least a day (preferably 2) from your last PE session before measuring or you will be measuring some temporary effects left over from your sessions.