Slide the 496mm 2040 extrusion into the channels of both 560mm 2040 extrusion. The 2 holes on the 20mm side that are closest to the top of the 560mm 2040 will be for the top of the frame. The top of 496mm and the 560mm 2040 should be flush.

Using your corner clamps, clamp both 560mm 2040 and the 496mm 2040. Tighten the M6 button head screws.

Remove the corner and clamps. Double check to make sure that after you remove the clamps the extrusions are still square.

Do this for both the left and the right sides

Note: The work space I had wasn't ideal. The extra extrusion piece was used to make sure the joints would be flush.