Tape plastic sheeting from the top edge of the wall, hanging down to the floor. Cover the floor with the plastic and tack it in place to keep it from moving around while you work on the ceiling. Remove any smoke detectors and wrap any light with plastic and turn off the power until you are finished with the room.
Score the ceiling lightly with the scraper if your ceiling texture has been painted over with oil-base paint. You want the moisture to be able to get under the paint to release the material. Don't scrape so deeply that you hit the drywall of the ceiling.
Pour very hot water into your sprayer and add a small amount of dish detergent into the reservoir after the tank has been filled. Set the spray nozzle so that it emits a fine spray. Put on your goggles to protect your eyes.
Mist the ceiling until it is shiny and wait about 15 minutes. The moisture should be absorbed by the ceiling material during this time. Give it a test scrape and if the popcorn texture still seems dusty and dry underneath, spray again for extra moisture.
Scrape the textured material with an 8-inch scraper. Continue spraying with a mist as you work if the material seems to be sticking. The popcorn ceiling that was never painted should scrape off easily while painted surfaces might need more work.
Mist the area after all the scraping is done to help settle the dust. Wait for it to drop and then remove all the plastic from the floor, working from the outside edges to the center, wrapping the old popcorn ceiling material inside. Leave the plastic on the walls until you have repainted the ceiling. Throw away the balled up plastic in a trash bag to keep the ceiling material from falling out.