Hold the 290mm extrusion, vertically, against the end of the 359mm extrusion.

We use the 290mm extrusion as a spacer to correctly set the position of the joining plate.

With the help of the second joining plate, hold the plates as shown in the 2nd image. Ensure that the joining plate is flat on the table and flush with the 290mm extrusion before fully tightening the M5 screws.

You can now remove the vertical extrusion and verify that the plate is placed like on the 3rd image.