Okay, memento gave good directions, but I’m going to attempt to be a little more thorough….
First of all, aluminium rulers work best. They’re firm enough to press into the pubic bone with, but also flexible to allow for the rulers surface to run along an upward or downward curve.
Bone-pressing can be painful, it is recommended that you place a thin piece of cloth between the zero-end of the ruler and your flesh. I usually just keep my boxer shorts on and press the ruler into them.
The spot you choose to measure from should be chosen carefully as you will need to measure from that point to accurately measure gains. I use a spot on the top of my penis slightly to the right of the center. When choosing a spot, do not attempt to go under the pubic bone, this will be painful and is pretty much considered cheating.
After choosing your spot and while standing, press the ruler firmly to your pubic while your fully erect penis is horizontal to your body. If you have a curve and are using a flexible ruler press the ruler down with your other hand at the approximate center point of your penis. Attempt to look directly down at the ruler when recording the measurement so that your line of sight doesn’t play tricks on you.
Keep in mind that average BPEL is between 5.5-6 inches.
As for being obsessed with size, well, if you weren’t concerned with yours, you wouldn’t be here =D.
I’d like you to seriously consider PM’ing me about any further questions you might have or if you’d just like some advice. PE is really a wonderful thing, you’re lucky to have found it.