To remove the popcorn ceiling, start by protecting all the surfaces including floors and walls that you do not want to get damaged or wet. Since water is used in the process be sure to turn off the electricity at the breaker. Use a garden sprayer or spray bottle if you don’t mind refilling often and saturate the ceiling. Use a plastic scraper to remove the popcorn material. As a side note if your ceiling was installed in the early seventies it may contain asbestos. Send a small sample to a certified lab prior to removing. While removing the popcorn, chances are some the joint compound was also removed. To insure a flat ceiling refill the joints and any screw/nail holes with fresh joint compound. Let dry, sand and repeat, to insure a flat surface a skim coat may need to be applied. Prime the ceiling with primer recommend for wallboard and a blank slate is created.