Remove furniture from the room in which you're working or place it in the center of the room and cover with drop cloths. Place drop cloths or tarps on the floor and remove fixtures, window coverings, switch plates and outlet covers. Use painter's tape to protect windowsills, baseboards, door hinges, the ceiling perimeter and anything else that you don't want to get paint on.
Use a sponge or cloth to wipe down the walls with mild detergent and water and allow it to dry. If walls have small cracks, holes or other damage, apply spackling compound with a putty knife. Once it dries, sand with fine-grit sandpaper. For extensive drywall damage, see Patch Drywall Holes to see how to make larger repairs.
If you need to make repairs, add an extra day to your plans to give the compound time to dry.