Guide on adding a new model, animation, and creature to HoMMV

Wonderful... I always dreamed of getting into modding, I told myself five years ago. Because it's so great, and errors never occur!

True. So true.

Added 3 minutes later
Regarding skinning. If the model isn't too different from the Heroes of Might and Magic III model, you can copy the skinning from the Heroes of Might and Magic III model to the new one. And make minor adjustments where needed.
Regarding animation, if the skeleton is from the Heroes of Might and Magic III model and hasn't been changed, you can use animations from the game.