Do a scrape test
Before you go to all the trouble of prepping the room, try scraping a small area. Try it dry first, then dampen the texture with water and try again. Some texture comes off easily without water, but in most cases wetting is best. If the water doesn't soak in and soften the texture, the ceiling has probably been painted or paint was added to the texture mix. In that case, wetting the ceiling may not help, and you'll have to decide whether you want to tackle a really tough scraping job or choose another way to hide your popcorn ceiling.
Test for Asbestos!
Any popcorn ceiling installed before 1980 might contain asbestos—a known cause of lung cancer. Before trying to scrape off any popcorn texture, contact your local health department and ask about getting a sample tested. If the test comes back positive, cover the popcorn with new drywall or tongue-and-groove planks, or hire an asbestos abatement contractor to remove the popcorn.
Prep for a big mess
Cover floors and walls with plastic drop cloths. Don't use canvas drop cloths because water can soak through. Cleanup is easier with plastic too, because you can just ball it all up when you're done working and throw it in the trash. Leave the plastic in place after scraping to catch the mess you'll make repairing and sanding the ceiling later.
Get the furniture out
If possible, remove all furniture from the room you'll be working in. Scraping popcorn is messy work, and you won't want furniture in your way every time you move the ladder around. If moving everything out of the room isn't possible, cluster it and cover it with drop cloths.
Remove ceiling fixtures and fans
You might think it's easier to leave light fixtures and ceiling fans in place, but they'll just be in your way and get covered with wet popcorn. Plus, you don't want to accidentally spray water into an electrical fixture.
Protect can lights from water spray
If you have recessed 'can' lights, stuff news?paper or rosin paper inside them to keep them dry. Also, make sure the power to those fixtures is turned off at the circuit breaker panel or fuse box.
Wet it with a pump sprayer
For easier scraping and practically no dust, use a garden pump sprayer to mist the ceiling and let it soak in for about 15 minutes before scraping. Only give it a light misting—too much water could damage the drywall or loosen the joint tape. If the texture hasn't softened after 15 minutes or so, spray it again and wait another 10 to 15 minutes.
If the texture still hasn't softened, it might be painted, or paint might have been mixed into the texture before application. In either case, water won't easily penetrate. If the texture is painted, you might be able to dry-scrape it first to expose some of the unpainted texture and follow up with wet scraping. If the texture has paint mixed in, you might have to dry-scrape the whole ceiling or cover it up with drywall or T&G boards.
Cover electrical boxes
Shut off the power to any electrical junction boxes in the ceiling and cover them with painter's tape to keep the wiring dry when spraying water on the popcorn. Overlap the sides of the junction box with the tape, and then trim around the perimeter with a utility knife, being careful not to nick the wires.
Work in small sections
Only spray and scrape a small area at a time—about 4 x 4 ft. If you work too large of an area at once, the popcorn might dry before you have time to scrape it off. If that happens, respray the area and wait another 10 to 15 minutes before scraping.
Tame the mess with a mud pan
Use a mud pan—the kind for holding joint compound—to catch the wet popcorn before it hits the floor. That way, you're not tracking it all over the place when you walk and move the ladder around. Also, use the edge of the pan to clean off your scraper when it gets loaded up with wet popcorn. A mud pan is available at home centers.
Round off the corners of your scraper—whether it's a wide putty knife or drywall taping knife—so it won't gouge the ceiling and leave you with dozens of ceiling wounds to repair. Use a file, a sander or an electric grinder to do this.
Scraping alone won't leave you with a paint-ready ceiling. You'll probably have small dings and gouges to fix. At a minimum, you'll have to sand the ceiling to get it perfectly smooth before painting.
Here are some tips on fixing drywall and getting it ready for paint.