It's easy to install the Gmail Add-in through the Chrome Web Store extension. Simply use Google Chrome Web Browser and follow the steps below.
Video Tutorial
Instructions
1. Locate the Gmail Add-in in the Chrome Web Store using the following link:
You should see the following after clicking the link above:
2. Click on the Add to Chrome button.
3. Click on the Add Extension button to confirm.
4. Return to your Gmail page and reload(refresh) the page.
5. Click on the Delivery Trust® Settings in the top right corner.
6. Click the LOGIN button and sign in using your Delivery Trust credentials or sign in with Google.
7. Click on Compose to start a new message and you should now have the Delivery Trust® Protection Toggle.