Pour the tequila, triple sec or Cointreau, apple juice, and sugar-free syrup (if using). Add a handful of ice cubes, stir with a spoon, and refrigerate for one hour.
Serve & Garnish:
If you enjoy a rimmed glass, prep the rim of your glass by rubbing a lime wedge along the rim and dipping it in cinnamon sugar.
Fill each prepared glass with a few ice cubes, pour some of the margarita mixture into the lowball glass, and top it with 2 ounces of sparkling water or club soda.