In this guide, we are presuming you know the basics of how to find files on your harddrive and we are describing it out of a Windows 8 perspective.
Before you start
Before you start you will need some addons to install! A good place to look is ESOUI. Download one or several addons to a location on your computer where you can find them again.
Step 1: Locate "My Documents"
First we have to locate "My Documents" - There is usually a shortcut to it under "This PC".
In your "My documents" folder, you will find a folder named "Elder Scrolls Online".
In this folder, you will have one or two folders called "live" and/or "liveeu".
The "live" folder is for the NA client, and the "liveeu" is if you are using the EU client.
Step 2: Create or locate "AddOns"
If you haven't got a folder called "AddOns", one has to be created. To create a folder, you right-click in the empty white space and choose "New -> Folder". It is very important that you give it the correct name "AddOns".
Once the folder is in place, we are ready to install our addons!
Step 3: Open the downloaded ZIP-file
Since I am lazy and prefer to drag and drop, I usually open up another explorer window, by using a keyboard shortcut (WIN+E), locate my downloaded ZIP-file, double-click it and it will expose the addons folder.
Step 4: Install the addon
You can now do the final step in two different ways:
Double-click the "AddOns" folder to open it up, then switch back to the other window and drag that folder into the the addons folder that is open in the background
or Drag and drop that folder on top of the "AddOns" folder and that will make it extract the files there.
Step 5: Launch the game and enjoy your addon!
You are now done and can log into your game. IF you did all this when you were still online, you have to refresh your addons in-game by pressing ESC and then choose Addons -> Reload.