Wet the mildewed area with water. The water will wet the mildew and stain to prepare it for removal.
Mix some of the poultice with a bit of water until it is the consistency of peanut butter. Mix enough poultice to cover the mildew.
Spread the poultice onto the mildew so it is 1/4-inch thick. Avoid spreading it on unaffected areas.
Cover the poultice with plastic wrap and seal it to the granite with tape. Use a pin to prick a few holes in it so the poultice can dry. Let the poultice sit until it is dry -- up to 48 hours.
Scrape off the poultice with a plastic scraper. Rinse the area with fresh water and dry with a soft towel.