1. Install Undertale Modding Tool[github.com]. Despite the name, this also works for Nubby's Number Factory as they both use the same engine (GameMaker)
2. Begin by locating your game's data.win file. This is in your steamapps folder, the location depends on OS:

• Windows 10/11:
  C:/Program Files (x86)/Steam/steamapps/common/Nubby's Number Factory/
• Mac:
  ~/Library/Application Support/Steam/SteamApps/common/Nubby's Number Factory/
• Ubuntu:
  ~/.local/share/Steam/steamapps/common/Nubby's Number Factory/

This may vary if you have changed any settings, but 99.9% of the time it is in the listed locations.
3. Duplicate the data.win file, and rename the one of the files
4. Open the unrenamed file with Undertale Mod Tool
5. You can now begin modding the game.

I have very little experience with modding NNF as of now, I would just like to share how to start it, look for an UTMT guide. I will add onto this when I figure things out.

WIP