To import the shader into your project, append it as you would when linking any material from another .blend file. For more info on appending, check here: https://docs.blender.org/manual/en/2.80/files/linked_libraries.html#append-and-link

It is recommended to use Branched Path Tracing with this shader.

The shader is simple in function, taking only a few inputs. I'll explain what each one is for here:

• The Base Color input does what you'd think: it takes an input colour or texture. The shader uses this information to figure out the colours for all the SSS layers and the diffuse base. 
• The Subsurface Amount Multiplier slider is equivalent to the SSS weight slider on a Principled BSDF node, and is used to control the strength of the SSS. 0.1 is the default, and multiplies the SSS by 1 (baseline strength). 0 turns off the SSS, and anything above it turns it on. Consider the strength of the SSS to be multiplied by the value of this slider multiplied by 10, i.e. 0.1 = 1, 0.5 = 5, etc.
• The Melanin Offset slider offsets the brightness/lightness of the skin tones. In most cases this can be left at 0, but can be tweaked to improve the accuracy of the skin tones produce. Higher is lighter skin, lower is darker skin, and 0 is the baseline. 
• The Roughness input is a regular, PBR specular roughness slider. Use it as you would for any PBR shader. 
• The Normal Map input is an input for a normal map. The strength slider below this controls it's strength.
• The Auto Bump Mix Slider mixes between the input normal map at 0 and the automatically generate bump map at 1.
• The Auto Bump Strength slider controls the strength of the Auto Bump.
• The Auto Bump Distance slider controls how "deep" the generated bump map is. 
• The Auto Bump Inversion Map input mixes between a non-inverted Auto Bump map at 0, and an inverted one at 1. This is useful for things areas on the input colour texture like hair, which are dark, but proud of the surface, not indented, and so produce false bump results. This input allows the the user to input a mask to choose which parts of the Auto Bump map are and aren't inverted.
  
  Hopefully that explains the shader fully! If you have any other questions, feel free to message me here.