Once you have made the Google Analytics add-on available for your account (or reseller account), they will see it under their Marketplace tab. When they click on Manage they will see the following pop up asking them to Install the add-on:

To complete the installation, click on Install this Add-On

To edit the add-on, click on Default Add-on Settings and enter your Google Analytics tracking code in the space provided

You will see results and keep track of your interface in your Google Analytics account