Run a lime wedge around the rim of a glass. Dip the edge of the glass in salt and fill with ice.
Add a few ice cubes to a cocktail shaker and add the tequila, orange liqueur, lime juice, and sweetener (if using). Cover and shake to combine.
Strain the margarita into the prepared glass and garnish with a lime slice.
Notes
To sweeten your drink, you can add more agave syrup or simple syrup to your liking.
Don't have a cocktail shaker? You can stir all of the ingredients in a separate glass, shake them in a mason jar, or simply pour all of the ingredients into the prepared iced glass. Shh! I won't tell.