Re-mark the pencil lines with temporary pieces of masking tape/painter's tape set about 8 inches from the ceiling/wall joint. Once the tape is up, notice that the two sides of the ceiling that run parallel to the ceiling joists will offer only a thin strip of ceiling joist for securing the crown molding. The other two sides of the ceiling and the walls wear tape in regular increments all the way around. You will add the first piece of baseboard to the edge of the ceiling that offers only the thin strip of joist.