How do I hang drywall where the wood is uneven?
Went through this recently myself.

I'd try to limit the joist-to-joist out-of-flatness to 1/8" or less.  If it's an obvious part of the ceiling, try for 1/16". 

Get a long straightedge and a pile of drywall shims.  High joists are pretty easy: add shims to bring them down to flat with the others.  Low joists require you to build up the adjacent joists gradually over a few joists to meet that 1/8" (or 1/16").

I attach the shims with staples, but many people like to glue ceiling wallboard in place, to prevent dimples at the screws/nails in the future. If this includes you, then the shims should also be securely glued.

Once you get the drywall up you can somewhat correct ridges and valleys with a repeated skim coats of joint compound.  However, it's difficult, messy, and much easier to fix the framing first.

36" Drywall Shims: (menards.com)