Cover the floor of your room with painting drop cloths.
Stand on a stepladder so you can reach the ceiling. Sand or scrape any raised areas, dried paint drips or bumps off your ceiling using sandpaper or a razor knife.
Apply a thin layer of lightweight joint compound to the ceiling using a taping knife. Allow the compound to dry overnight.
Put on a dust mask. Sand any rough edges or raised spots.
Repeat Steps 2 and 3 one or two more times until you are satisfied with the smoothness of the ceiling.
Tape the walls at the edges of the ceiling using painter's tape. Use a paintbrush to paint around the edges.
Pour ceiling paint into a tray. Use a roller to apply it to the ceiling. Let dry for 24 hours, then apply a second coat if desired.