First, the music itself wasn't written by me, but by Rob Miles[1]. So I had a version in C available. I then iteratively transformed the code into simpler and simpler expressions, and finally into a simulated assembly language, written as C macros[2]. Only the final step, initializing peripherals, stetting up interrupt handlers, etc was done with the actual chips. Of course, I made some erros with the before mentioned C macros, so some final debugging was trial-and-error. Later on I also used simulators, but they don't support all the necessary features of the MCUs, or were outright broken[3] (patches now upstream).
[1]: http://txti.es/bitshiftvariationsincminor
[2]: https://git.gir.st/Chiptunes-pms150c.git/blob/f1b013452400b0...
[3]: https://sourceforge.net/p/sdcc/patches/379/