For general add-on management, refer to the Blender 4.2+ Manual for newer extension platforms. Add-on Tutorial - Blender 5.1 Manual
While the request specifies version 3.1, modern versions (such as V3.3.2) have since been released to support newer Blender versions. How to Install the Add-on
Search for "Serpens" or "Visual Scripting" in the search bar and check the box next to the name to activate it. Resources & Documentation