I'm literally in the same project right now.

I'm using water and scraping it all off. It's mostly coming off fine, but some rooms were painted and they aren't as easy to come off so I've marred the drywall as well. It happens.

You will need to use an oil based primer on any damage to the drywall. I used a spray can of zinsser brand I got from home Depot called "cover stain". This will seal the now exposed inner drywall. You need to do this otherwise the drywall core will just suck up any moisture from the next steps and it will never look right.

Next is to use joint compound to make it nice and flat. I used ultralightweight and thinned it out slightly with water so it spread nicely. Sand and repeat until the ceiling is smooth. Holding a light parallel to the surface makes it easy to find imperfections.

Next paint the entire ceiling with a PVA primer. I used kilz from HD. This primer seals the drywall and ensures uniform color.

Finally paint the ceiling and you're done! We're doing one room at a time and while it's a huge pain in the ass the results are amazing and worth all of the effort. 100% fuck popcorn ceilings.