ᐅ Install a dropped ceiling by attaching wooden battens to the joists.

Hello everyone,
I am currently working on installing a suspended ceiling in an older building with uneven walls. I have mounted beams on the walls and placed several cross beams between them.

My first question is: Where is the best place to start with the first batten to which the cladding will later be attached?
And my second question: How can I ensure it is level so that I don’t have problems with the uneven walls and the cladding?

For the cladding, I plan to use gypsum fiber boards measuring 120x60cm (47x24 inches).

Thanks in advance.