Adding New Models From Recent Expansions

Hey there, so I recently found the post with MPQ files that add Legion models to characters, new looks for the mounts, etc.

So after getting the basic ones I kinda got addicted to it and now I'm wondering how I can make and add some of them myself.

Basically I want to change all the boring reskins to something more unique, for example change the gryphons. One of them into Spectral gryphon, one into Shackled ur'zul, and another one into Tyrael's Charger, and so on. There's hundreds of new mounts that could potentially be used.

I found a couple of posts about how to swap models but they were all old and most of the links were dead. So I figured I could ask here, if anyone has links or knows how to do it, please post them and thanks in advance. :D