Make sure your pi-top  is switched off
Find the green Display Cable shown here
You don't need the HDMI or USB-A adapters.
Plug one end of the cable into your pi-top and the other to the back of the screen
Plug in the Power Supply Unit and power up your pi-top 
Slide the switch to the left until the green light comes up!
You're all done!
Once your pi-top  has powered up, you'll see this screen on your FHD Touch Display