Hang ceiling, attach wooden slats to beams

Hello everyone,
I am currently in the process of suspending the ceiling in an old building (slanted walls). I have mounted beams on the wall and placed some cross beams in between.

My question now is: Where is the best place to start with the first batten, to which the cladding will be attached later.
And my second question: How do I make it straight so that I don't have problems with the slanted walls and the cladding?

For the cladding, I want to use 120x60cm gypsum fiber boards.

Thanks in advance.