Navigate to your campaign by clicking on the "Edit" button in your Dashboard.
In the left navigation, click on the "Installation" button, then "Custom website" as shown in this screenshot.
Copy the line of code as shown in "Step 1" of this screenshot.
Install this script on your website site by pasting in before the closing </body> tag of the page.
Refresh the home page of your website. Then refresh this page. If installed correctly, you should see a green check here.