Protect the area you are working in with plastic sheeting to make clean up easier. Lay the sheeting down on the floor, then tape it all along the walls using painter's tape. Cover electrical outlets with plastic sheeting and painter's tape. Turn off the electricity to the room you're working in.
Fill an empty garden sprayer with warm water. Use a ladder to reach the ceiling and spray a 5 square foot section with the water. Make sure the section is completely wet, but don't saturate it.
Scrape off the popcorn texture with a 6-inch putty knife. Apply pressure to get off the texture but don't press too hard or you risk damaging what's underneath.
Continue this process, wetting a 5 square foot section then scraping off the popcorn, until all of the texture is removed. Spray more water on any spots that are tough to remove.
Wipe down the ceiling with a damp rag to remove any residue.