To get the smooth contouring effect seen in the example you provided, I usually start with a very softened version of the photo, and then apply filter / artistic / cutout in order to give me an initial guide. I would mask out the eyes, mouth, and any other details that I didn't want to contour.
View attachment 48186
To improve this, I then would use the pen tool to produce my own paths which would roughly follow the contours produced by the cutout filter, but which could be smoother or more angular, contain cusps (like this example does), etc. One could also produce new contours in-between those generated by the automated process described above.
I would then turn off the contours and filled areas produced by the automated filter, and would fill each new path that I generated with successively lighter or darker shades of gray, as needed. I didn't demonstrate this because this is the most time-consuming step.
HTH,
Tom M