Creating new ANIM with replaced textures

[Desblat]

I am trying to create new mob ANIM based on default one. Like a spider but with custom texture.

I know it's possible, because I saw mods with such a thing, but I can't figure out how to by myself.

I tried to replace altas-0.tex inside the ANIM zip with my custom atlas, and renaming "build" using BinaryConverter, but it doesn't seem to work.

Is there any tutorial on it or something?

Thanks in advance