When using a customer facing screen you must connect it to a register. This will allow the customer to see their order being built and swipe / pay on their side of the counter.
To do this, on the register tap the settings icon, and then LinkWith. You will then see a view showing all devices on your network running the application. Tap the drop down below the device listed at the top (your current device) and set it to register.
On the screen you will be using for a customer facing screen; Go to the same area, and set it as a customer screen.
In the list below, tap the area listing your register, and then tap connect.
This will connect your devices. On your register screen you will then see the connection to the customer screen listed.
In the event that you need to move or change connections you may do this easily. From the register screen, go to LinkWith in the settings menu. Tap the "x" between the devices you wish to separate.