First, go to Sites and find the site you created earlier:
Click on the + Ad Unit button on the right side of the page to open a new pop-up window:
In this window, you can enter a name and title for your Ad Unit and customize its settings, such as the ad thumbnail image, color, margins, and currency. If you want to test your ad unit in a sandbox environment, make sure to turn on the switch in the bottom-right corner.
When you’re finished setting up your ad unit, click the Save button in the bottom-right corner.
You’ve successfully created your first ad unit. To learn more about testing your new ad unit, refer to this document.