Not really a tutorial, but here are the steps I take most of the time when I make a digital painting.
1. Here I drew my picture out with mechanical pencils and grey copic markers and then I scanned it into my computer. (Sometimes I also just do the beginning sketch with my wacom.)
2. I do this- [link] and I begin to paint in my base colors.
3. Once I have all my base colors, I merge the base colors with my sketch and then I start painting on top of that. I begin to add details and I merge layers when I decide they are how I want them to be. (I don't like working with a lot of layers. I like a more traditional approach where you are just adding on top of what is there.)