Ventilate the room by opening doors and windows to help air it out, since fresh air will help remove pet odor and also reduce paint odor when you're painting. Remove any furniture as well.
Spray any existing pet stains with a solution of warm water and white vinegar in a half-and-half solution. White vinegar is a very effective stain remover and a natural deodorizer that will help dissipate odor. Soaking up the vinegar an absorbent cloth will help remove both the stains and the odor.
Vacuum any carpets in the room to help remove hair and skin cells in the fibers that contribute to pet odor.
Lay down plastic drop cloths to protect the floor from paint.
Spackle any dents or holes in the walls by spreading the spackle with a putty knife over the wall damage in an "X" motion. Let it dry overnight and then sand it down using 150- to 200-grit sandpaper to smooth it into the wall. Wear a dust mask when sanding.
Use masking tape around room borders and door frames that could potentially get painted by accident to protect them.
Spray a primer on the walls with a paint sprayer to help cover up any preexisting paint as well as provide a substance that will help the paint bond with the wall surface. You also can use a roller if you prefer it over the sprayer.
Clean out the sprayer with warm water to completely remove any remaining primer before adding paint.
Add a coat of paint to the walls once the primer is completely dry. Latex or oil-based paints work best, depending on your personal preference. Apply a second coat once the first coast is dry for best results, since it will make the paint job thicker and more durable. The paint also helps lock in odor underneath the coat and will assist in removing stubborn pet smells.
Prevent any pets that tend to urinate indoors from entering the room. They are likely to urinate in the room as a way of marking their territory in response to the new smell of the room.