Place a strip of painter's tape on the wall of the room needing crown molding. Get the painter's tape as close to the ceiling as you can without taping the ceiling.
Place another strip of painter's tape directly below the first strip. Make sure both pieces of tape are touching but not overlapping. Peel the strip of painter's tape closest to the ceiling off the wall. You now have a strip of wall left that needs to be painted.
Paint the section of wall just above the painter's tape with a brush. Be careful not to paint the ceiling. If you are concerned about painting the ceiling, place painter's tape on the ceiling to avoid painting it.
Allow the paint to dry. Place a strip of caulk at the top of the painted strip and a strip of caulk at the bottom of the painted strip. Allow 24 to 48 hours of drying time. The caulk adds dimension to the painted strip and is more realistic looking.