Since nappa leather is porous and absorbs water easily, avoid getting them wet. Getting the shoes soaking wet can irrevocably stain the leather.
Most quality shoe repair shops will offer products to polish, protect, and refresh the leather's surface. A neutral cream polish—one without color—will also help protect the surface from scuffs and scrapes and keep the leather hydrated. If something's spilled on the leather, try using a soft cloth with a small amount of cleanser or cream on it. Rub the surface in a circular motion, then allow the shoe to dry before buffing the entire thing lightly with a dry, clean cloth.