How Do I Level Drywall If 2X6 Aren't Flush/straight?

I am finishing a remodel that we ran out of money for. It was professionally framed, but costs to do drywall were out of site, so will have to do myself. Have a little experience on another home, biggest problem has been screwing the drywall to the 2X6 then finding out the boards were not fully flat. Also my home is late 1800s and has a "swirly" texture and coved ceilings/arches. I tried to imitate...not much luck..looks very home-done!