General Music Hacking Guide

But what I want to know if I were to take say, a song from CT/Silent Light, and replace it over the FF6 cave theme (like what GiNattack did with the Eternal Crystals hack), what exactly do you do to place it in the right sequence?

First of all, JCE3000GT is responsible for injecting Chrono Trigger's Silent Light into FF6 that replaces Umaro's theme, and has released the patch for it. I had nothing to do with it, or Eternal Crystals for that matter, that hack belongs to MetroidQuest.

Now you see, the way MetroidQuest is doing his custom music is by literally custom composing it, rather manually sequencing the music data using a hex editor and the help of a midi sequencer, and that is what this tutorial here explains. It is incredibly challenging, and requires some actual knowledge about musical notation.

The way JCE3000GT is injecting music into FF6, is by finding Squaresoft games with similar SPC cores, like Chrono Trigger, Romancing Saga's, Radical Dreamers, etc. These games use the same type of commands and code for the music that you will see in the FF6 song data. But while similar, they are not the exact same, and still requires a lot of know-how and experience to properly cross the songs over, it is no easy feat in itself, but is certainly easier than composing a custom song from scratch by referencing a midi sequencer, or sheet music.

Do you copy and paste it in a text editor and then edit it accordingly with a hex editor?

No, that usually will not work. You might hear parts of the track, but it is not that simple and would most likely come out sounding bad, because the games use slightly different commands here and there, and you must consider that If you wanted to copy/past (insert the binary data) from one track from another songs data, it would have to be the same size or smaller. I have had mild success with crossing some songs tracks over with this method, Ruined World from CT is one that comes to mind, but only a very select few songs will sound good. Pretty much every song will need more work than just copy and pasting one track, to another.

You must estimate the size of the sequence data and create the length-specifying bytes when you insert it. If the data exceeds that of the original song data, you will have to put the new song data where there is enough free space, and then change the pointers and instrument sample indexes and all that. Hopefully JCE3000GT can explain how he does it better when he gets back.