Like the other guys, a lot of reading, a lot of asking questions and from those that have used and done so seriously. After that, a lot of trial and error, and then more asking. Some of the best stuff I've learned has come from Dave and Milos.
I enjoy when some of the people you're refereeing to ask the question, "Are you a doctor." Most people don't understand some pretty simple facts. Most doctors don't know that much about AAS, most will tell you this if they are honest. They don't spend years studying the affects of these drugs while in medical school, in the grand scheme of things there isn't that much info out there that would be considered medically legitimate for them to devote a lot of time to. But that's only because our society has chosen for there not to be.
I'm not saying there is not good medical info out there, there is, but there isn't tons and tons of info that is acknowledged by the medical community as a whole.
Another thing I thoroughly enjoy is when some guy will try to take some simple advice that I give and refute it with some article that he probably has a very limited understanding of that came from a so called "Expert." In this case they will twist the words to fit their own ideas and say look, this guy has "Expert" attached to his name so there. Many of the so called "Experts" out there have the same base of knowledge that the rest of us have (rest of us meaning those that have tried to gain some knowledge about this.)
Myself, I've never once tried to claim I'm an expert. I have never given anyone medical advice or tried to play doctor with their health. I give out my advice based on some of the actual facts that we do know, based on what's worked for me and others as well, and I try to base this advise and hand out my opinions based on a surrounding level of truth to the subject. After that the person can make their own judgment.
So know, I'm not a doctor, I haven't made my own gear in my house...never will. How deep is my knowledge? More then most but less then many.