Custom music

That tutorial is good for once you get a hold of the music data, whatever it may be, but in order to get your own custom song data you'll need to do the main music hacking endeavor, which there are 2 different ways to go about it:

Harder, tedious manual way: https://www.ff6hacking.com/forums/showth...ic+hacking
Much easier, yet still challenging at times MML way: https://www.ff6hacking.com/forums/showth...l+tutorial

There's a bunch of song hacks done here also if you didn't know that you might find use for:
https://www.ff6hacking.com/forums/showth...p?tid=2558
This is what that first tutorial you linked was made for, mostly.

Anyway you should get a better idea of the process from those links there.