Make sure the horizontal data flow arrows are all pointing in the same direction and vertical orientation arrows are pointing up. Now is the time to check. If these panels aren't aligned properly you will have to disassemble the entire frame to correct it.
Loosely screw in the corner brace onto one side before placing the next. Make sure the corners are flush and tighten the screws once everything is aligned properly.
Repeat this for 3 corners. In order to fit the receiver card, there's no room for the 4th as we'll see in the next steps.