Drop 6x M5 T-nuts into the frame channel at the back top left.

Make sure these 9 screws are not completely tight. In reality you only need to tighten them enough so the screws can grab the threads and not fall out.

Place the Left Corner assembly on to the back top left corner of the frame

Thread in 2x M6x10 button head screws into the 2040 first. Tighten them until they're snug than back off 1/4 turn. We do this in case we need to adjust the alignment of the top plate

Thread in 6x M5x8 button head screws into the T-nuts in the frame channel until they are snug. Loosen them 1/4 turn