XChat

Download. You can get the latest version for free at silverex.org, or you can download the shareware version at the XChat website.
Install. Open the file you downloaded and proceed through the installer. Here is what the white background theme looks like; leave it unchecked and you'll get the black background theme. Otherwise, leave the options as they are and you will be fine.
Customize. Open your newly installed copy of X-Chat. In the X-Chat > Server List, enable Edit mode to begin customizing. Select UnderNet in the Networks column, and edit the rest to your liking - here is an example.
Chat! To join a channel, simply type /join #channel. For example, to join the Bar & Grill, type /join #rs_bar_and_grill. Now you can just start talking!