Starting from the front of the machine, clamp the first rack module using the assembly gauge to secure its position.
Fully tighten the 8 screws of the rack module.
Now bring the short rack module and use it as a gauge to create a perfect space between the first and the second rack modules, as shown in the animation below.
Fully tighten the 2 first screws of the second rack module.
Again, use the assembly gauge to clamp the second module and fully tighten the 6 remaining screws.
Replicate the same operations a third time on the last module, and then again on the other side of the frame.
Animation
Take your time to align everything nicely here, it will influence the precision and performance of your machine.