Outlining, coloring and shading
Having created basic shapes I start to put rough color inside.
Instead of focusing on one particular detail I prefer to work on the entire image. For that reason, there is a bit of color everywhere.
At
this point, let me explain my choice of “Pen Pressure Control” for “Opacity Jitter” and “Flow Jitter” in “Other Dynamics”. Basically, with those settings I can easily achieve a nice color blending. Using only “Pen Pressure Control” for “Opacity Jitter” it's much harder to achieve nice transitions between colors. Here are some examples of the effects that I have achieved.

Using “Pen Pressure Control” for “Opacity Jitter” and “Flow Jitter” transitions are much smoother to achieve with less brush strokes:

Apart from the above, there are really no more tricks; just a lot of patience, close examining of the real photograph and painting, painting and once again painting.
Let's come back to filling image with colors:

I still have my sketch visible on the top layer to get filled all shapes:

At this point I am going to turn off the sketch layer and start working only with color, shading and highlights:

I slowly shape the image by adding shadows and highlights:
