Juice the limes and lemons. Mix them together with the Cointreau. Refrigerate until ready to serve; lasts up to 1 week. Cut a notch in a lime wedge, then run the lime around the rim of a glass. Dip the edge of the rim into a plate of kosher salt or flaky sea salt (or for a festive look, use our Margarita Salt). Pour 3 1/2 tablespoons of margarita mix into a glass and top off with 3 tablespoons tequila (about equal parts). Or, add the entire mix to a pitcher with 1 1/2 cups tequila. Serve with ice.