Wash with mild soap and warm water and a soft cloth, rinse with clean water more than once (leftover cleaner causes streaks), blot off the excess and let it air dry. The coating is what makes vinyl waterproof, and cleaning is about protecting it: never let a bleach solution soak on it, never use undiluted bleach, and never apply a fabric treatment. For mildew, use a diluted mildew remover made for marine surfaces, then rinse repeatedly. Let sunscreen soak in before anyone sits down.