Unfortunately...

It takes a very practiced hand to skim/sand a very smooth, beautiful ceiling. It's just the brutal truth. When people look ACROSS a ceiling, it's obviously seen in a different "plane" than looking AT a wall. Waves, lumps, bumps, & tool-marks are easily seen.
.....this is obviously why textured ceilings are often done. It covers a "multitude of sins", and is fairly cheap.

The people who are good at it WILL charge for it, cuz it AIN'T easy! Done well, it adds to your homes value though!

Faron

If there's a stud behind it, you can try screwing a drywall screw into it and see if it pulls it up if it's sagging drywall, but you'll need a stud-finder to know if that's an option. That's really the only hope other than patching it (just like you do on a wall, but it has to be PERFECT). Ideally, cut the whole area out (half sheets of drywall are easier to blend in than small patches).
You can sometimes put in cross studs by toenailing them in to the existing studs to attach a smaller patch piece. That's a good solid fix. Or you can float in a long piece between studs (second video, hard to describe).
Here's the right way to do it:
https://www.youtube.com/watch?v=3NYcnjTPunk
Here's a cheating way you MIGHT be able to pull off if it's less than a 2 foot area:
https://www.youtube.com/watch?v=xQIWXf5BtaE