First thing you will want to do is fork the repo so you can make a pull request.
Then you should clone it recursively to your computer so you have all the needed extra stuff.
(Make sure you have a 2019.2.x version of Unity)
After that, everything is stored as prefabs. Items are pretty simple, but creatures are quite a bit more involved and honestly pretty annoying to do currently. I need to really work on the way they're loaded right now to make them easier to mod.
Anyway, items. All item prefabs are stored in "Resources\Items" with the prefab the same as the item token. Items that can be modded, such as weapons and armor, are each in a subfolder, for example "Resources\Items\WEAPON\ITEM_WEAPON_AXE_BATTLE"
Each item prefab needs to contain a single mesh, with the "Item Model" script added onto it, and a material with one of the "Building/Something" shaders, with mostly self-explanatory texture maps that would be the same as any other PBR material, except for a 4 color mask, which determines which parts of the texture should take the texture/color from the material it's made from.
The mask channels are as follows:
Red: 0 = Material texture and color, 255 = your own texture.
Green: 0 = Your own texture, but colored by the material color, 255 = your own texture. (This may be replaced some time in the future, because nothing uses it)
Blue: 0 = Blend the material bump map and smoothness maps with your own, 255 = your own bump and smoothness maps.
Alpha: 0 = color everything according to the wearer's job color, 255 = Don't do that. (This is mainly for clothing, but it's supposed to work with any held item)
AO maps can be used, but are not effected by the mask.
Let me know if you have any other questions.