Last modified: 1/9/2022
So, you're modding Guitar Hero II, and you want to...
songs.dta
For further reading, I'd highly suggest you take a listen to (and read the companion slides of) Eric Malafeew's talk at GDC 2005 on Harmonix's homebrew scripting language. The knowledge will serve you very well if you're doing a deep dive into custom scripting.
.chart
For further reading (especially while I get this section written up), check out the C3 authoring documentation. It's for Rock Band, not Guitar Hero, but the stuff about authoring guitar is still fairly applicable.