Pass Var from child window back to parent

I'm not sure about how to do this one...   Here is what I am after:

User is on a window - lets say to fill out an order
User needs to select a customer - I open a new window (pop-up) to find customer
User selects customer on the pop-up window

How do I close the pop-up window, and pass an int value back to the parent order window, and somehow call a method using that int value?

Let me know if this is confusing.