Actually, getting started is quite easy. You need a KDE2.1-aware version of Brahms, which can be found in the kmusic CVS module. There is also information on how to get Brahms on the aRts Homepage in the Download section.
When you start it, it will show up in the midi manager. If you want to do synthesis, simply add a synthesis midi instrument via Add+aRts Synthesis Midi Output. Choose an instrument (for instance organ2). Connect them using the Connect button. Finally, you can start composing in Brahms, and the output will be synthesized with aRts. It is usually a good idea to have the artscontrol window open, and see that the volume is not too loud (quality gets bad when the bars hit the upper limit). Now you can start working on a new aRts demosong, and if you are done, you can get it published on aRts-project.org ;-).