The files you want to be looking at are data/raw/items/armor.json and data/raw/recipes.json. The majority of what’s in there at that point is pretty self-explanatory; just copy something similar to what you want to make (for example you could copy the chitin armor) and then change the values on it to match the ones you want.
Note that the armor won’t spawn naturally (that requires editing the code itself right now), but you will be able to craft it.