Begin by removing as much furniture as possible, cover what remains with plastic drop cloths or buy a small roll of visqueen plastic. Begin by wetting the texture and we have found that using a small pump up type garden sprayer makes this task a lot easier.
After 10 minutes or so the texture will become very soft. At this point you can take a 6″ to 12″ wide drywall finishing knife and scrape the texture off. Hold a small trash can or bucket under the knife as you scrape so that the texture drops into the bucket.
Another procedure is to tape the tool to a 4″-5″ roller pole or push broom handle and scrape from the floor allowing the texture to fall on the plastic covering the floor. When finished bundle up the plastic and toss the whole mess in the trash. Avoid wetting an area more that 2-3 times as you do not want the drywall board itself to become saturated resulting in sagging.
Allow the ceiling to dry thoroughly and then lightly sand using a sanding pole and screen cloth. Screen cloth is an open mesh material that will smooth the surface without abrading the paper of the drywall board. Prime the ceiling using (preferably) an oil base primer or if latex is your choice use one of the stain/seal variety’s like KILZ-2 or Zinsser 1-2-3. Allow the primer to dry and then apply a coat of flat white latex finish paint.