Leather is naturally soft, but if you use the wrong products to clean your sofa, the leather can become dry and brittle. Water and leather conditioner are best to clean your sofa, because the conditioner softens the leather and removes stains.
Dip a soft cloth in water, and wring out as much water as you can. Gently wipe the damp cloth across the surface of the leather. Once the water dries, apply a leather conditioner to the surface.
For severe stains, combine 1 tsp flour, 2 tbsp baking soda, 1/4 cup sea salt, and 1 cup warm water. Wipe down the entire stain, and then dry it with a clean rag. Spraying ink stains with hairspray and blotting with a soft cloth should remove ink from the leather.
If you leave water sitting on the couch without drying it or using a leather conditioner, you risk developing water stains on the leather.