Cover the floor with a heavy drop cloth. If the floor is carpeted, tape the drop cloth to the baseboards all the way around the room to seal off the carpet from the invasive dust.
Wear goggles that seal around your eyes, and wear a full respirator that comes with interchangeable filters. The little fiber respirator masks are not sufficient for this job.
Seal off adjoining doorways with taped-on plastic drop cloths, and open a window.
Fit a large-pad pole sander with a coarse sanding screen. Sanding screens come in various sizes; a coarse screen will have large holes, and a fine screen will have smaller holes.
Start in one corner of the ceiling and sand off the texture by rubbing the flat pad back and forth. Ceiling texture is simply dried joint compound, and it will come off in billowing clouds of dust.
Sand with vigorous strokes, maintaining even pressure. Sand as uniformly as possible from one side of the ceiling to the other side. Don't worry about sanding it perfectly right now --- you won't be able to see the ceiling clearly. Let the dust settle after you've sanded the entire ceiling one time and then inspect your job. There will probably be rough patches, but most of the texture should be gone.
Spray the rough patches lightly with plain water and sand them carefully with a pole sander that's fitted with a fine sanding screen. Sanding damp texture does not produce as much dust, so you can see where you're sanding. However, it's only effective for small areas, since it also dampens the drywall, which is more likely to suffer gouges when it's wet.
Shine a strong light on the ceiling after the second sanding to locate any residual texture, and sand these areas smooth one at a time. Wipe down the ceiling with a tack cloth before you paint to remove any dust that's sticking to the surface.