Remove all objects from your desk. If applicable, move the desk away from the wall so that you have room to work.
Screw the metal brackets to the back of your desk. You may wish to mark your bracket locations before screwing them in. Use a pencil and a meterstick to make sure that your marks are even.
Clip or bolt the cable tray onto the brackets. Wiggle the tray around a little bit after it has been attached to make sure that it is connected securely.
Replace the contents of your desk. Run the associated electronic cables up from the wall socket or power strip. Lay them within the cable tray.