Well, doing it is simple. Find something that has DXM and NOTHING else in it. Eat it. Go temporarily insane.
My ex-fiancee and the guy she cheated on me with began chemically cutting pure DXM out of robitussin in their dorm rooms and did rails of the stuff by the gram. Most of them got kicked out of school for it, and even after a friend died, they still fuck with it. Prolonged use/abuse of larger quantities causes permanent dementia and numerous organ failures.
Anyways, last I heard about her, her personality was completely changed, and she's now railining yay that her rich lover buys her like there's no tommorrow. The next time I hear anything about her, it'll probably be an obit.
Nevertheless, you guys know I would never give you shit for anything you take (as I am a recovered opiate addict, myself). All I'm saying is that its not too wise to do this shit every weekend.
As for dosages, I'm not sure what exactly is considered adequate. After what I went through with seeing it, I've never personally done it.