Attach one finished side plate to the X axis C beam and drag chain rail, with 15mm screws:
And then: roll the X carriage on the C beam:
Before attaching the second finished side plate (and also attaching the leadscrew to the X axis motor) – note that you can also add t-nuts (marked as number 8) to later add reinforcements to the structure:
If you want, you can also add reinforcement to the structure now – this is optional: