Prepare the following parts:

1x 359mm extrusion + plate assembly

1x 290mm extrusion + plates assembly

3x M5x10 screws

3x t-nuts. Slide them in one side of the 359mm extrusion as shown in the picture.

Attach both assemblies together with 3x M5x10 screws. Do not tighten them fully yet.

With the help of a square, make sure you have a 90° angle and fully tighten the 3x M5x10 screws.

Make sure the plates and extrusions are oriented as shown in the picture!