With the main tones on, now I paint to make the tones smooth. What I used for this picture is the Water Diffusion tool found in the Water Color submenu. It's kinda like cheating because spreads the paint out and adds water automatically, but who cares. As long as you get the effect you want, no? See, it looks a bit better now.

I clean up the mess at the lines now and here's a close up. The smoother you want it, the more you go over the colors with the Water Diffusion Tool. Water Diffusion interacts heavily with the Grain property in the Brush Control box. Always turn the Grain to 0 or else Water Diffusion will leave very visible grain marks on the surface.

With the main tones in place, I add more detailed shadows to the picture. I picked a blackish tone to add more depths to the shadows.

The final touches of the newly added tones went through another bout of Wet Diffusion. This is how the shoulder looks after that process. Significantly smoother.