Measure and cut all the pivot arm components and drill the pivot arm side rail pivot hole as per the details below.
Use a 16mm bolt or shaft to align the holes in the side arms with the pivot plates, including the compression tubes, and clamp into position.
Tack weld the plates in place and run a weld around the compression tubes to secure them in position. The tubes do not require a full perimeter weld.
If you have room in your workshop, you can tack weld the front, corners and sides together and put to one side, otherwise, leave as separate items until the main chassis has been fabricated.