Welcome to a small tutorial on where to shade, it might be kind of hard to see on this skin but I will explain you where to shade anyways, download the skin to see the pallette I used too.

Head:
The lower part of the head (Chin, neck, etc.) should be dark, this is logical because no light reaches it.
Under the hair you need a slight dark layer aswell because of course, hair blocks the light so causes a bit of the shadow, the ears also need some shading to kind of show the form of the ear.

Torso:
You need to shade the left, right, up and lower sides of the torso dark, this is because it isn't reached by a light source.
Put a dark edge around the front and back of the torso aswell, this is mostly to create a round shape and to make the skin tile better.

Arms:
The arms need to be shaded dark on the right side (You can see in MCSkin3D!)
Also, depending on the clothes, you need to add a dark layer of shading under the sleeve, and the same to the edge so it tiles again.

Legs:
Here you need to shade the right side dark again, this is the area inbetween your legs where no light comes (Hurr hurr!) Also the upper layer of pixels under the torso need to be darkened, because your shirt/sweater/hoodie, etc. create shadow on your pants (If there).

I hope this helps you out a bit, cya next time!