Wipe the cushions on all sides using glycerin soap and soft, clean cloths.
Repeat, and wipe away suds until your cloths come away clean. The leather should feel instantly more flexible.
Set the cushions aside to dry.
Wipe the rest of the couch, regardless of its dryness, to ensure that the entire couch has a consistent color and appearance.
Clean the leather with a commercial product, if you prefer. Homeowners use everything from Murphy’s Oil Soap to Windex, but professionals stick to careful formulations from companies like Lexol and Leather World products (see Resources). These are very mild, like skin cleansers, and contain little or no drying agents such as alcohol.
Apply a commercially available leather conditioner from a high-end maker such as Lexol or Leather World Technologies. These contain small amounts of oils, along with penetrants; they add enough oil and lanolin to the leather to restore it, but not enough to make the surface tacky or oily.
Apply several light coats of conditioner with clean cloths. Apply to every inch of surface, including the cushions, arms and backrest, to ensure a consistent color and consistent protection.
Allow the conditioner to dry and penetrate completely, which should take two to four hours.
Apply a protectant, which acts like wax on your car or a water repellent on your clothing. Once again, use a commercially available protectant from a quality maker, which will be formulated to allow leather to breathe.
Apply two thin layers of protectant to every surface, paying particular attention to armrests, headrests and cushion surfaces.
Use vinyl cleaner on any “leather match” (vinyl) surfaces, which will likely be the sides and back. ArmorAll and Murphy's Oil Soap are perfectly suitable for this.