Entering "cleaning sails" into a search engine will get you a great deal of mostly good advice, but the usual method is to lay the on a tarp, get a bucket of mild detergent and a soft broom, and get down to some work. Then hose off and hang to dry. That will shift normal grime, but mildew and rust stains need more specialist treatment.

All best avoided by having dark coloured sails, IMHO.