To add a Box to a Template, hold down the shift key, and then drag the Box over to the template. As you do this, you will see the Container Boxes are now highlighted in pale yellow.
To place your new Box into a specific Container, wait until it turns a brighter shade of yellow; and then “release” the Box onto it (by letting go of your shift+drag action).
Once the Box is inside the Container, you can simply drag it into the precise position you desire.
Click Save Template to preserve your changes.