Prepare the walls for painting. Patch any holes with a putty knife and paintable spackling, and even out the surface. Wash the surface of the paint to remove all dirt and any oils left by fingerprints. Allow to thoroughly dry.
Sand the surface lightly with fine-grit sandpaper. This will knock some of the "shine" off the walls created by the semi-gloss paint and make it easier for the new paint to adhere.
Prime the surface with a coat of primer paint. Use a stain-killing primer if there are stains remaining on the wall to keep them from bleeding through. Repaint the surface once the primer has completely dried.
Add a second coat, if needed.