The guts are basically this... the game has a particular model type (say henchmen in this case). Then the game has a large number of art definition files that say what a henchmen model will support or what particular looks it will take. For example, when they made the female breastplate vs. male breastplate, they added things for the female that the male just doesn't have. In turn, I had to go through every piece of armor (which is a lot) and every item (which is not so much) and all the abilities (not so much) and add a new model type for the female henchman. In turn, this is how I produced the female henchmen mod that I currently have available.
So, when LH is released that will be one of the first things I port over, since my snaking mod already works and my bridge mod already works. I'll look at bringing over my swamp tiles over and a few other things and making things compatible. But first I'm looking at getting a complex quest editor up and running. I think I've got the guts working and most of the annoying bugs out. I was able to produce a 6 objective quest in a matter of minutes. It was rather nice. Got a few more things to add in there.
There is nothing that explains this core modding really except what heavenfall has produced. But I just looked and guessed and tried and saw.