Take your time with this step.

Use the build_helper_z to set the correct distance, from the back of the frame to the Z axis.

Place your square on the Y axis extrusion, as shown, and push in the direction of the build_helper_z.

Ensure that the Z axis extrusion is perpendicular to the Y axis and fully tighten the 3x M5x10 screws.

This step can be made easier by using a clamp. You can then secure the thicker side of your square to the bottom extrusion as shown in the 3rd image.

As your clamp may be too thick to fit under the extrusion, raise the frame on blocks or a plastic part (we used the psu_upper_mount in the photograph).

Check that the Z axis is at 90° and adjust if necessary

Verify that the plates and extrusions are correctly oriented.