How to install an add-on that is not in the Kodi.tv repository using an add-on zip file.
In this example the hybrid repository is being installed, but the method is the same for add-ons. First download the ZIP file, then:
Settings → Add-ons → Install from zip file -> Navigate to the ZIP file
Step 1: Select Install from zip file.
Step 2: Navigate to the ZIP file and select it.
Step 3: In the bottom right, Kodi notifies when the add-on is installed and enabled.
Step 4: Select Install from repository. The new repository is listed. In the case of add-ons they are added under All Add-ons in the appropriate category.