Remove all the furniture possible in the room. Dismantle or cover light fixtures and ceiling fans.
Drape the walls with plastic, using duct tape to attach to the uppermost walls. Overlap seams to ensure removed texture cannot penetrate. Spread plastic over the floor, taping the wall plastic to it as well.
Spray a heavy stream of warm water, from a large squirt bottle, on the ceiling. Cover 2 or 3 inches of the surface with water and allow time for it to penetrate. If the water soaks in and the ceiling material becomes damp and gooey, you have a sand-textured drywall mud finish. If the water beads up and refuses to soak in, the finish is sand-texture paint.
Mix a solution of warm water and trisodium phosphate, at a ratio of about 1 quart of water to 1 tablespoon of TSP, and fill your spray bottle for water-soluble drywall texture. Open a can of gel-like chemical paint stripper for use on textured paint; the consistency makes it ideal for ceilings overhead. Pour a portion of the remover into a glass jar and work from this container.
Spray the ceiling with water to saturate. Alternatively, use an old paintbrush to apply the chemical paint stripper. Work small areas of several feet in dimension. Apply a thick coat of either product, following any special instructions on the product if using a stripper.
Use either a floor scraper or a putty knife to scrape the ceiling surface after about 30 minutes has passed. The larger the scraper surface, the more you can scrape at one time. However, ensure you're not gouging the ceiling material as well, especially if you work from the floor with the floor scraper. Spray additional water or chemical stripper on as you work, if the finish resists removal.
Work into the corners with smaller tools. Consider using steel wool as necessary. Continue with additional ceiling sections until the surface is completely stripped. Finish by sanding and refinishing as desired.