A PDF document detailing use of this shader is included with each download, but a quick start guide is available here:
To get started with your shader, simply open your Blender scene, and either:

• Add Dermis from the asset browser: 
  Blender's asset browser is very powerful when set up properly and used with a selection of asset .blend files. Both .blend files provided in Dermies are already set up to leverage this. All you need to do to add Dermis and/or Dermis Lobe to your asset browser within Blender is to add them to the asset folder that you have set up within the Blender preferences. If you do not have this folder set up, please see the documentation here: https://docs.blender.org/manual/en/latest/editors/preferences/file_paths.html#asset-libraries. Once you have this folder setup, and the Dermis v1.3.blend and Dermis Lobe v1.3 files added to your new asset folder, Dermis should appear in your asset browser.
• Add Dermis or Dermis Lobe by appending it from Dermis v1.3.blend or Dermis Lobe v1.3, either as a node group, or as a material.
  Each .blend file you create has its own internal file structure, and this file structure can be accessed and read from within other .blend files. One way to do this is through appending: https://docs.blender.org/manual/en/latest/files/linked_libraries/link_append.html#append 
  Once you have navigated to and opened your chosen Dermis .blend file in the Append dialogue box, you will see a set of folders within the .blend file. The ones you are interested in are Material and NodeTree. Inside the Material folder you will see a material named "Dermis Shader", if you are appending the full Dermis material, or "Dermis Lobe", if you are appending just Dermis Lobe. If you select this file and click Append, the default Dermis material within .blend file will be added to your current scene file, alongside any other materials.
  Inside the NodeTree folder you will see a node tree called "Dermis Multi-Level SSS" or "Dermis Multi-Level SSS Lobe". If you select this option and click Append, the default Dermis/Dermis Lobe Node Group, NOT material, will be added to your scene file, alongside any other node groups. This node group can then be added to any material like any other shader node from the shader editor's "Add" menu, either at the top of the shader editor via the shift + A hotkey. 

Once you have brought Dermis into your scene by one of the two above methods, you will see a node group that looks alot like a standard Principled node, but with extra controls. These controls are described in detail in the documentation PDF, and in Blender 4.0 and above, by tooltips that can be viewed by hovering over each control. All inputs that are also present on the standard Principled can be used as normal.