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.
Step 1: Place down base colors
But a quick warning first: do this at a very high resolution. The effect will be completely lost otherwise. It doesn't have to be this big, but the final image I showed you is 4200x2000 pixels at its full size. Just save it to a smaller size for the net later.
For this particular example I'm using a shirt sleeve that had a lot of detail in the wrinkles because it shows the effect well.
Because of the nature of the specific details of the area, I did not use my tablet for this part but instead just used the polygonal lasso tool to put down the shades. For more organic areas like arms and the face I paint out the shades/highlights with my tablet.
I've included an example of a base done with a tablet in the images.
I create a folder for each general color needed, in this case that includes folders for Blue (hat, undershirt, wristbands and such), Light grey (shirt), Brown(glove), and so on.
This technique needs to have a high level of contrast to work properly, so needs somewhat detailed shading. Small areas may be able to get away with the base color, one shading color and one highlight, but larger areas generally need two degrees of highlight and 2 degrees of shading, as well as pure black/pure white touch-ups.
In the case of the sleeve, I used a base color, one shade, and two degrees of highlighting. And then did a little more that I will detail in a later step.