Prep the Trim
Place a drop cloth along the perimeter of the area you will be painting to catch any excess paint or dust created from the sanding process.
Use fine sandpaper on trim that is being repainted to remove any excess paint and to smooth areas that have been damaged. Gently rub the sandpaper over the surface in a back-and-forth motion. Feel the surface to ensure it is smooth and even.
Caulk any holes or spaces between the trim and the wall. Smooth the caulk with your index finger or using a damp cloth. Allow to dry.
Attach painters tape around the perimeter of the trim. Place it above and below the trim (along the wall and the floor or ceiling).
Paint the Trim
Shake the can of oil paint before opening to ensure the paint is mixed appropriately. Open the can of paint and pour a portion of it into a separate empty can (about 1/4 of the paint). This will make it easier to work with.
Use an oil-based paint brush. Dip the brush half way in the oil paint, knock the brush on the sides of the can and apply to the trim.
Begin at the top of the trim, allowing the brush to overlap the lip of the trim. Finish the coverage over the surface of the trim. Do not apply the paint too heavily or it will run, creating a mess to clean up.
Turn the air condition or heat on in the house to remove moister from the air, increasing the rate the paint will dry. Allow to dry. Most oil paints can take anywhere from eight to 12 hours to dry. Apply a second coat if needed.
Use paint thinner during the painting process to clean up small messes. Water will not work to clean oil based paint.