@craftyguy IMO the worst thing about Debian packaging is that the answer to your question is: It Depends™. Every maintainer is free to do things their own way.
If you're lucky and the package in question follows DEP-14, you'll most likely easily get there with gbp: https://wiki.debian.org/PackagingWithGit
If you're unlucky, you'll need to possess enough understanding of everything to react to what you stumble upon there. Which isn't rocket science, but isn't beginner-friendly either - RTFM required 😉