Check for lead-based paint. Be absolutely sure that the paint you are covering is oil-based and not lead-based. If you are unsure whether the paint might be lead-based, take a chip of it to your hardware store to be tested. Lead-based paint may require professional removal.
Prepare your working surface. Put down dropcloths and scrape the wall thoroughly. Remove any stubborn chips and as much of the original paint as you can. It's a good idea to wear a dust mask or respirator to avoid inhaling paint dust. Sand the surface down until it is completely smooth. If working on a tough exterior wall, consider using a power disc sander to remove the old paint. Brush off any excess dust.
Apply the primer. If you intend to cover the surface with acrylic paint, you can prime with acrylic primer, but you will need to use at least two coats of primer. Let each coat dry thoroughly before applying the next. If you plan to apply new oil paint, prime the surface with a coat of alkyd primer.
Re-paint once the primer has dried completely.