Cover the floor with 6 mm plastic sheeting. Tape the sheeting around the room at the base of the wall with painter's tape. Set cardboard over top of the plastic to prevent slipping.
Cover the walls with 6 mm plastic sheeting. Tape the sheeting to walls where the ceiling meets the wall with painter's tape.
Pour one teaspoon dish soap for every gallon of water into the detergent compartment of a pressure washer. Spray the popcorn ceiling on a low setting with the mixture, allowing it to soak in for 20 minutes. Keep the ceiling damp for the 20 minutes by reapplying the mixture.
Scrape the popcorn off the ceiling with a 6-inch wallboard knife. Avoid puncturing the ceiling with the knife by holding the knife level and gently scraping. Remove all popcorn.
Sand the ceiling with fine-grit sandpaper. Patch holes with joint compound applied with a putty knife.
Remove the plastic and tape from the walls. Prime then paint the ceiling if desired. Remove the cardboard and plastic from the floor.