Open Realband, then make sure your MIDI and Audio drivers are set to the same thing as BiaB uses.
This is done in the Prefs area (big button in middle of screen)

Once this is done, just open a BiaB song in RB and hit play. You'll see how it gets laid out.

From there it is pretty intuitive; highlite a new blank track, plug in your input to the computer that you are using to record, and hit Record. You will record on a new track. RB puts everything right in front of you for audio.

Look thru the Realband Tutorials listed here -
http://www.pgmusic.com/howtos.htm
There should be a pdf file in your Realband folder that may help also.

Then ask anything you need help with in the Realband Forum

I do not work here, but the benefits are still awesome
Make your sound your own!