Remove all furniture from the room. Cover the floor with a canvas or plastic drop cloth. Cover doorways with plastic sheeting—held in place with painters tape—to contain the splatters and plaster dust within the room where you are working. Remove cover plates from light switches and wall sockets to keep them splatter-free. If there is a lot of damage (nicks, cracks, large holes), you must repair these first. You may just need to finish joints between pieces of new sheetrock; maybe you have to finish broken plaster or a plaster-sheetrock joint; or perhaps you plan to repair plaster that has begun to crack from years of settling or vibration. Perhaps you simply want to flatten your popcorn-style ceiling.  Pull any nails from the surface that will receive the skim coat. Fill in the holes with joint compound. Cover any cracks in plaster walls by scraping out any loose plaster, filling the hole with joint compound, and applying joint tape to prevent the crack from spreading. Allow to dry before continuing. Dust first, then wash if necessary to remove any grease. Use a sponge or a damp towel to wipe down the surface. Use water or a wall-friendly cleaning product depending on the extent of any grime. Rinse the wall with clean water after using any cleaning product.  Sweep away loose particulates with a duster, or go over the wall with the dust-brush attachment on your vacuum cleaner.  Wipe light smudges with a damp, clean sponge or paper towel. For more resistant grime, try wiping the wall with a mix of warm water and mild detergent. Try rubbing stains with a paste of baking soda and water. Try mixing 1 cup ammonia, 1/2 cup vinegar, and 1/4 cup baking soda with one gallon of warm water for a powerful homemade cleaning agent. Consider using commercial surface-cleaning products like 409 and Pine-Sol. You should only skim over matte paint or primer. Any other painted surface should be primed with a basic primer, then prepared by wiping down with a degreaser. This allows the compound to attach to the surface and not slide off or bubble. If you have removed wallpaper from a wall, prime the surface again with an oil-based primer.
Protect furniture and entryways from dust and splatter. Determine the damage to your walls or ceilings. Thoroughly clean the walls or ceiling. Apply a water-based primer/sealer to surface, and allow to dry.