Note: Asbestos was used in some sprayed coatings for ceilings from 1935-1978 and can cause health probllems. It’s a good idea to have a sample tested by an NVLAP approved testing center before getting started. If there is any asbestos, experts recommend you do not try to remove the popcorn yourself. Either have it removed professionally or cover it with a new drywall ceiling.
Remove all the furniture the room if possible, or cover it with waterproof plastic sheets. Spread plastic sheets or tarpaulins on the floor to protect it.
Put on a long sleeved shirt, wear a hat and make sure your eyes are covered. Safety goggles with sides are best.
Use that spray bottle and water to moisten an area of 2 to 3 square feet. Alternatively, you could us a long nap ceiling paint roller dipped in water. The objective is to wet the popcorn enough to soften it and make it "easy" to scrape off without soaking the ceiling above.
Once a section has softened, use an 8 or 10 inch taping knife to scrape the popcorn off the ceiling.
Move across the ceiling, section by section, moistening and scraping until all the popcorn is gone.
Once the popcorn has been removed, sand the ceiling and paint it. You will probably have to Spackle some sections of the ceiling, some maybe more than once, to get it as smooth as you would like.
A Couple of Pointers
Putting too much water on the popcorn could cause water spots on the underlying ceiling drywall, something that can be just as unattractive as the original popcorn texture. Try not to catch the corners of the taping knife. This can create holes in the ceiling that will need to be repaired.
An alternative method to wetting and scraping would be to use a sander and dry sand the popcorn. While this technique would probably work, it would create all kinds of drywall dust (a very fine powder) that would spread throughout your house, and probably rule this method out as a viable option.
Rather than going to the trouble of either wetting/scraping or sanding, a third alternative would be to simply install a new drywall ceiling over the popcorn. You would lose about 1" of ceiling height but it might be the most hassle-free way of getting rid of that popcorn.
Source:
http://www.doityourself.com/stry/paintpopcornceiling