Start by removing anything possible from the room. Protect the floor and anything that had to stay in the room with a thick sheet of plastic. Lay out some old newspaper on the floor on top of the plastic to soak most of the wet mess up.

A note of caution – if your popcorn ceiling is from the early 70's it may contain asbestos. If you suspect your ceiling may be from that era, remove a small piece of the popcorn ceiling and send it in for testing. If it does contain asbestos, it will have to be removed by professionals who can also dispose of it properly to reduce the cancer risk asbestos can pose for you and your family.

Wear some old clothes, a hat to keep the goop out of your hair, and protective eyewear. A respirator of some sort or at least a mask to keep most of the dust particles out of your lungs is a good idea as well.

Use a spray bottle and completely saturate part of the ceiling. Let it soak in for a few minutes and then use a scrapper tool to scrape the gooey mess of your ceiling. A drywall-finishing knife works well. Hold a bucket under the tool to catch most of the removed popcorn ceiling. Work in small section and keep repeating the process until the popcorn ceiling is removed. Use a wet sponge to wipe off any remaining residue on the ceiling.

You may have to rewet some parts of the ceiling to get the popcorn to soften up enough to remove it. Try not to wet it more than 2 to 3 times. You don't want the drywall beneath to get too saturated with water. It may warp or grow mold.

Once the entire popcorn ceiling is removed, let the ceiling dry completely. Now it's time to sand the entire surface to remove any rough patches. A pole sander works great for this. Give the dust a chance to settle and then use a joint compound to patch any wholes, cracks etc that may have formed on the ceiling. Let the compound dry for about an hour, then lightly sand over it to smooth it out.

Once you get the mess on the floor cleaned up your new smooth ceiling is ready to be painted.