Clean the leather surface before working any oils down into the surface of the material. Start off with a damp cloth and gently wipe the surface down to remove dirt and debris.
Clean the leather with leather soap as directed by the manufacturer to remove stains from the material. As a general rule, this means simply dampening the surface with a rag soaked in the leather cleaner, gently scrubbing at the surface with the rag or a scrub brush, and then wiping it clean and allowing it to dry.
Select a leather polish or oil for your couch. Commercial products are available from home improvement stores, or you can use natural products such as linseed oil, beeswax or neatsfoot oil. Bear in mind that each type of polish or oil will darken the color of the leather somewhat, so always test an inconspicuous area first to make sure you'll be happy with the end result.
Apply your leather polish to the surface of your sofa with a rag. Work it into the surface with a rag to ensure even coverage. For best results, rub with the grain of the leather and apply more than one coat to ensure higher levels of polish.