The first place to look for tutorials is the tutorials database here:
https://forums.totalwar.org/vb/local_links.php?catid=70
In your case, look in the Skins (textures) category:
https://forums.totalwar.org/vb/local_links.php?catid=75
I recommend these two:
Tutorial: a guide to skinning (beginner)
Tutorial: a guide to skinning (intermediate)
Bookmarks