Probably the kind of thing "painter" would be used for, but heres what I do in photoshop.
This is very simple, but I've been asked a few times what precisely I did to get the look, so I thought I would share it.
Of course, while the technique is still simple, using it in practice takes time. Its not a filter or anything.
For each step I will provide the simple step, and then go into detail on some organization and details of my process for anyone who wants more detail. If you want the 20 second version, just read the parts above the separation line.
warning: This is intended for use with a graphic tablet (wacom in my case). You can try it with a mouse but I don't know many people who can do much digital painting with a mouse.
And I'll note that I am far from a professional, and merely have done some of this in my free time. I do not pretend to be an expert, just simply wanted to throw this out there.
The image below is a finished digital painting I made using this technique, although you cant see the purpose of the technique at this size. Read on for that.