Take your time with this step.

Take the smallest extrusions of those remaining. It is 290mm long. It will be used as a reference, to ensure that parts are correctly spaced.

Place the assembly at the end of a table, as shown, so that the two 331mm extrusions lie flat on the surface.

Carefully place the 290mm extrusion between the other extrusions, as shown in the 2nd image.

Be careful that you do not scratch or permanently mark the extrusions whilst doing this.

Apply pressure in the directions shown and fully tighten the 2x M5x10 screws.

Carefully remove the 290mm extrusion and reserve it for later.