1. Place 1 (2-gal.) zip-top plastic freezer bag inside another 2-gal. zip-top plastic freezer bag. Combine first 3 ingredients in the inside bag. Seal both bags, and freeze 24 hours. (Double bagging is a precaution to avoid spills.)
2. Remove mixture from freezer 1 hour before serving, squeezing occasionally until slushy. Transfer mixture to a 2-gal. container. Garnish, if desired.