It is highly recommended to check the end assemblies for squareness before proceeding

End assembly

Place an engineers square on the lower 3030 extrusion as shown

Carefully slide the square towards the corner

If the connection between extrusions is square, there should be no gap between the long edge of the square and the extrusion next to it

If the connection is not square, loosen the M6x12 screws connecting the corner plate to this extrusion, adjust in the directions shown and then check for squareness again

Repeat this process for all four corners on the assembly and then repeat everything for the second end assembly