The best way to learn is to download XRNS files, look at them, and edit them for fun.
Here’s a few links:
http://www.trotch.com/mbc/
http://www.renoise.com/beatbattle5/entries.php
http://www.yomirecords.com/music/botb/renoise/
https://forum.renoise.com/t/keith303-jitter/22271
https://forum.renoise.com/t/acid-trip-squelch-mp3s-and-xrns/22256
Etc…