As a PC user who knows their way around the majority of the game files, here’s what you need to know:
The steam file for darkest dungeon is where the meat of it is all at. Audio is currently inaccessible to modders, so any new heroes or enemies or skills will be soundless. Pretty much anything can fuck your game up if you miss-code it properly, although the game has become more stable with modding than it used to be so the game will usually still work but just be glitchy if you messed something up. Pay very close attention to your additions/revisions to the coding, 80% of the content that isn’t core game stuff is just editable text files and pngs. The best way to figure out what bugs the game up beyond that is trial and error and asking more specific questions once you’ve hit roadblocks. I never modded anything in my life before DD, this game is extremely easy to mod even as it is right now. You won’t be able to really seriously mess with the game at a base level, but beyond that the sky is the limit.