What is the best way to build a wall when the ceiling is not square or plumb?

Same way you build one when it is.

You just have to take careful measurements and make adjustments so things fit.

I agree.
Top and bottom plates (studs), studs at each end, then fill in the middle with studs of different heights.
You can carefully shave drywall to fit or you can use a ceiling molding and a baseboard.

If the ceiling is sagging because of damage above, you can put as much or as little effort you want into fixing that first.