Skin is a delicate subject, anything that must resemble a human being is subject to quickly be rejected because it looks unnatural or fake.
This tutorial does not intend to provide the utmost realistic, indistinguishable from reality material, but rather the proper method for skin-like materials and a solid base for further tweaking.
This tutorial uses a free 3D scan head model that can be found here, along with its textures: http://www.3dscanstore.com/index.php?route=information/information&information_id=14
This model does not have the eyes or the shirt detached, so the skin material affects everything which will of course not be the case in a model specifically prepared for such use, so don't be scared by the look of the shirt or the eyes.
Start as usual with a default Arion BSDF material.
Now this is a crucial step. How do simulate skin properly?
Although you might be tempted to use the color texture into the Color channel and then add Sub-Surface Scattering, skin is that kind of material that typically gets (or should get) all of its color information from the light scattering inside the flesh.
Based on that observation, we won't start by the color part but by the surface shading and define the reflective properties of our skin material.
Reduce the Roughness parameter to the desired value (40% in our case) then lower the intensity of the reflection by using darker shades of gray for the Color and Edgetint channels (respectively RGB 30,30,30 and 80,80,80 in our example).
We also decided to pick a Fresnel IOR value of 1.3, as flesh is mostly composed of water (and fat). This is accessory but why not doing it.
Be careful with your reflection intensity, or your skin material will look too shiny and covered with oil. There is no need to have extreme reflections on normal skin.
Alternatively you can use a roughness, specular or glossiness texture (one is provided in this scan package) to control the Roughness, but that texture might require some tweaking in the Arion Filetex RGB modifiers to achieve the desired Roughness.
For the simplicity of this tutorial, we won't use any extra texture for now.
Now let's add some subtle extra shininess using Coating.
The Color and Edgetint channels were set to the same RGB values as for the BRDF attributes (RGB 30,30,30 and 80,80,80) but the Roughness level was lowered to 15%.
Now it's time to enable Transmittance,Sub-Surface Scattering and define a starting density for our skin material.
The Sub-Surface Scattering Coefficient parameter was set to 600 in this example.
Now we can start getting some color to the skin material. There are two different approaches here.
Many people create a specific Sub-Surface Scattering texture, usually an highly saturated reddish version of the skin surface color.
In the case of Arion Render and because of the way its physically-based attenuation and Sub-Surface Scattering works, we will use the regular color texture.
Why? Simply because the color texture represent the final result we want, so why use some borked colors textures?
Enable Transmittance Attenuation and connect the Male_UD_SSS_Colour.jpg texture to the Attenuation color slot, then adjust the Attenuation distance so the color seem right, particularly in the ears. In this example we used a value of 0.5cm.
Now what's left, getting the bump map in and fine tuning and eventually using the various specular and roughness textures in their respective channels and adjust your camera exposure, your lighting... (which I will leave you to it!).
Then a little post-processing and we get our final result.