MS Access XP

I have created a database for a department in our agency. I looked at the Northwind sample database and saw the Customer Orders form. I would like to replicate this type of form but am unable to figure how it has been setup.

Specifically I want to replicate how if you click on an Order ID in the upper subform the Details on the products included in that specific order are listed in the subform below.

I've viewed this form in design view and have replicated the controls but to no success.

Can someone explain how this can be accomplished.

Thanks