I am a strong believer of you don't need tutorial for handpaint, at least not those kind, those would mostly only teach you how to copy. To handpaint you simply need to learn your light and color theory, and basic paint skill, then it's just about practice.
good reference is the first step , just look up a lot of examples of handpainted things and approaches. also determine if you want to do the from scratch approach or from a baked highpoly.
that's the thing, handpaint isn't about technique or remembering stuff. There's 10 million way of doing rock. It's just like you were learning real paint, you don't copy someone else work, you do your own, you experiment with color and texture and light until it give something. Everyone i know that learn handpaint from…