With great difficulty, to be honest. A lot of the modules I looked at did not have great documentation. Even the docs for built-in/standard libraries was not very accessible to me, as a newcomer.
I think there's a bit of an over-reliance on in-code documentation. It's not a substitute for good overviews, tutorials, and examples.
A good idea to implement.
This was pretty invaluable for me early on, http://tryruby.hobix.com/
Beginning Ruby From Novice To Professional by Peter Cooper
Why's (Poignant) Guide to Ruby: http://poignantguide.net/
This is good for your first steps. Then check out The Ruby Way to dig in deeper:
http://www.amazon.com/Ruby-Way-Second-Addison-Wesley-Profess...