Stephen,
i know nothing abt the Iwata airbrushes, but one thing is the same no matter what you are using:
Practice!
get a sheet of cardboard, or some newspaper, get some paint in the cup and start experimenting w/ air pressure, trigger technique, distance, the whole ball of wax.
there may be some folks here who can give you some good info for that make & model, but you will still discover it isn't exactly right for you. maybe it is hotter or colder, more or less humid, you move the brush faster or slower, whatever.
i wish i could give you better help for the HP-B, but definitely start spraying paper!
good luck,
ed.