At the moment, songs are composed using a text editor. The entire composition process and file layout is described in the readme, which I strongly suggest you read before using the program! It can seem very tedious at first, but once you get used to it, it isn't that dissimilar to a regular tracker.
You'll have the most fun composing using the built in utilities; try using the "Record" program to record a sample, then truncate it, add effects or whatever you like, before bringing it all together in your trusty text editor!
Once a song is composed, you can play it by simply running "python3 unrefined.py filename.usf" from the command line. The utilities bundled with the replayer use raw, 8-bit unsigned files, but any type of sample can be used in a .usf file; 8 or 16-bit, signed or unsigned, mono or stereo, raw or headered - it all works!