Here's how to install Tend in Google Tag Manager
Click on your container
Click "add a new tag".
Click the tag icon
Click "custom HTML"
Copy and paste the Tend script in in the HTML field, then click the trigger icon.
Choose the "all pages" trigger so the script fires on all of your pages.
Click "save" and name your tag.
Click "publish" to publish the changes to your website.
Now your Tend script should be live on your site.