Yep, i used reference for this one (i put on a leather jacket and took some photos). I can't think of any specific tips for painting leather, but i found that after i painted it a couple of times with refs i managed to paint it decently without. (for example here: Legend of the Cryptids - Manalee ) So i guess it basically takes is a number of studies to understand a material, leather or anything else, and then you can apply it to various shapes without needing precise references.