How do I prep leather before conditioning?

Clean the surface. Rubbing alcohol or denatured alcohol are always a good first choice. These solvents are gentle enough for leather and vinyl and will effectively strip residue from oil, grease, many types of adhesives, and slimy residue from ArmorAll and detailer dressings. Water-based cleaner-degreaser-dewaxers are also an excellent choice. Do not use caustic or antibacterial cleaners. If in doubt, work in tandem with both water-based cleaners and solvents to remove an array of dirt, grease, and chemical residues. A soft nail brush will aid in removing dirt from the fine lines of the material's "grain." For suede and nubuck, use white vinegar.