Cleaning your dog’s leash is an uncomplicated exercise.  But before anything else, check the cleaning instructions on the leash as it may also entail special cleaning procedures. For most nylon leashes, they are machine-washable and perhaps you can just use a gentle clean setting. Air dry afterwards instead of putting in a dryer, which may cause shrinkage.  If it’s a leather leash, you can use cleaning oils specifically for leather. You can also lightly pat it with warm water and some soap, and leave it to dry as well.