I taught myself how to. i would just sit at the hookah lounge and just blow attempt after attempt after attempt. took me about 3 weeks of going there ((a total of about 10 or so nights at the lounge)) to get it pretty down pat.
The best advice i can give is to drop your jaw and make a nice big "o" shape with your mouth. LIke, try saying "minnesota". when you get to the "o" in minnesota, hold that shape and try to say "oooooooo" ((as in "oh" not "oops")) in the lowest pitch you can.
that's what i think about for mouth shape when doing it. other than that, just play with it a bit and see what works best for you