A little step by step I put together for a blog post (full process: https://josh-godin.squarespace.com/blog/ ). It details a process I use for production sketches, since you can update the light/materials/colors/etc without losing the linework or initial design shapes. Can be useful, depending on the habits of your art director ;]
This was also a test for my new Yiynova tablet monitor. After years of working on a Cintiq 24HD I can safely say, Yiynova is perfectly good!