Take your time with this step. It will be a reference for the rest of the assembly. A third hand might be useful, ask someone to help, if possible.

Make sure that you are working on a flat and level surface.

Place the assembly vertically, as shown.

Take care that the assembly does not fall.

Using a machinist's precision square, ensure that the two extrusions are perpendicular to each other.

Apply pressure in the directions shown and fully tighten the two M5x10 screws.

Tech tip: This step can be made easier if you have a clamp. You can then secure the thicker side of your square to the vertical extrusion as shown in the 3rd image.

Verify that the assembly is still square after tightening. Correct it if needed.