Cut 2 of the C-Channels along the 1/8" guide hole in the rails, so that you have two 27" C-Channels.
Attached the Corner Connects to the ends of 2 of the full length C-Channels using 1 3/4" socket head screws.
Attach the full C-Channels with the Corner Connects to the 4th and 5th holes from the ends of the cut C-Channels, so that that full length C-Channel's flanges are facing outwards.
(Note: ensure the end of the Corner Connects with 3 holes are connected to the full length C-Channels)
(Note: ensure the all of the C-Channel's center holes are downwards while assembling)