Old popcorn textured ceilings (also called acoustic ceilings) make a room look dated. Not only that, if they were applied prior to 1978, they may contain asbestos and be tricky to remove safely. Scraping the old popcorn texture off may not be an option, but installing new drywall right over the old popcorn ceilings is possible with the right tools and a bit of hard work.
Use a stud finder to find the ceiling joists and make pencil marks on the wall. This will guide you in lining up the sheetrock and attaching it to the ceiling.
Buy 3/8" or 1/2" sheetrock, enough for the job. Sheetrock comes in several thicknesses. The thinner pieces are most suitable for going over existing drywall and won't add too much weight to the ceiling.
You will need help installing the sheetrock on the ceiling. If you can't find anyone to help you, rent a drywall jack at any rental yard or home store. This is a simple hydraulic lift that maneuvers and holds the sheetrock into place while you install it. When hanging the sheets, stagger the seams and avoid having four corners butting together, as this creates a weak joint that will crack later.
Use drywall screws instead of nails, because they will grip better through two sheets of sheetrock and reduce the dreaded "nail pops" that often show up months after finishing a ceiling. A drywall screwgun will create automatic dimples by slightly countersinking each screw.
Fill your mud pan with plenty of joint compound (commonly called mud). Using the six inch drywall knife, apply a generous amount of mud along the first seam. Cut a piece of tape and using the same knife, press it smoothly and firmly along the seam. Remove the excess as you go. Do one seam at a time so the mud doesn't start setting up before you can apply the tape. Cutting the tape into the right lengths first can speed up the process.
Where the corner meets the wall, fold the tape lengthwise (it has a slight crease so it can be folded evenly). Apply a smooth coat of mud along both edges. Wait at least eight hours for everything to dry. In humid conditions you may have to wait as long as 24 hours for proper drying, since this is a fairly thick coat of mud.
Use the ten inch knife for the next application of mud. Before you apply the next coat, use the dry six inch knife to run over the seams, removing any dried lumps or excess. Apply the next coat of mud, keeping it as smooth as possible. Remember that too much mud will need to be sanded down, which is probably the worst part of the whole job. This is a thinner coat of mud and will take between four and eight hours to dry.
Apply a final, thin coat of mud in good lighting so you can see the imperfections. Wait until it is dry and sand it smooth where necessary. If you were very careful, the sanding should be minimal. The easiest way to sand ceilings is with a pole sander, which is a pivoting pad on the end of an extension pole. Wear glasses or goggles to protect your eyes.