Copied from here
https://www.esoui.com/forums/faq.php?faq=install
1. Download your chosen addon
2. Extract the files to a folder on your desktop.
3. Open your ESO folder.
If using a PC, the
typical default install path is: C:\Users\< username >\Documents\Elder Scrolls Online\live\
Or you can use the windows variable by pasting it into windows explorer: %userprofile%\documents
4. In the 'live' folder, create a new folder called AddOns (if it doesn’t already exist)
5. Open the AddOns folder
6. Copy the files from the folder on your desktop into the AddOns folder
7. Your final path should look like:
C:\Users\< username >\Documents\Elder Scrolls Online\live\AddOns\< AddOn A FolderName >\< files >
C:\Users\< username >\Documents\Elder Scrolls Online\live\AddOns\< AddOn B FolderName >\< files >
C:\Users\< username >\Documents\Elder Scrolls Online\live\AddOns\< AddOn C FolderName >\< files >