We don't have fancy tools for modding, yet, so it's a real old-school modding experience where you copy folders, edit text-files (all json), and add your own textures. You can then put the contents of your folder into a ZIP and upload it right here on mod.io, to share your creation with other players.
We have a part of our current modding system on GitHub, so you could even build your own tools and share them with the community for more comfort. So GitHub is also where we keep the primary documentation for modding. Here you go: