Take down window coverings, wall decorations, and remove wall plates from switches and outlets. Cover furniture and the area of floor to be painted with tarps or drop cloths. Repair any holes, cracks or flaws and give the surface a thorough cleaning. Tape woodwork and other surfaces not being painted. Prime new drywall, unpainted surfaces, or any dark colored surface you plan to cover with a lighter color.
When painting over an existing semi- or full-gloss finish, the surface must be sanded to give the new paint something to adhere to (paint adheres better to rough surfaces than smooth ones). Smooth over cracks and other surface imperfections with spackle and a putty knife, and then sand smooth. Viewing the surface from an angle will reveal any spots that may have been missed.