Combine 1 cup granulated sugar and 1 cup water in a small saucepan over medium heat. Stir constantly until the sugar is completely dissolved and the mixture is just simmering (do not boil vigorously). This should take 3–5 minutes.

Remove from heat and let the syrup cool to room temperature. If you want a thicker syrup for more body and sweetness, simmer 1:1.5 sugar to water (i.e., 1.5 cups sugar to 1 cup water) instead, then cool.

Once cool, strain the syrup through a fine mesh sieve into a clean container to remove any undissolved solids or zest, then refrigerate. Syrup will keep refrigerated for up to 2 weeks.

Carbonated Base & Flavorings (per 1 L batch)

Measure flavor concentrate

In a small measuring cup combine 2 tablespoons fresh lemon juice, 1 tablespoon fresh lime juice, 1 teaspoon vanilla extract, 1 teaspoon finely grated orange zest and (optional) 1 teaspoon citric acid. Stir to combine.

Add 4–6 tablespoons of the cooled simple syrup to the flavor mixture. Start with 4 tablespoons for a lightly sweet soda and increase to 6 tablespoons for medium sweetness. Adjust later to taste.

If using food-grade coloring, add a drop or up to 1 teaspoon to the flavor mixture and stir until evenly colored.

Assemble the soft drink

Place a large pitcher or two tall serving glasses in the refrigerator briefly to chill. When ready, pour 1 liter chilled carbonated water into the pitcher or glasses. Stir the flavor-syrup mixture gently to ensure it is homogeneous, then pour it slowly into the carbonated water while giving one gentle stir — avoid vigorous stirring to preserve carbonation.

Taste and adjust: if too tart, add more simple syrup 1/2 tablespoon at a time; if too sweet, add a squeeze of lemon or lime. If acidity feels flat, add a pinch (1/8 tsp) more citric acid and stir gently.

Garnish and Serving

Serve

Fill each glass with about 1/2 cup (approx.) ice cubes. Pour the prepared soft drink over the ice, leaving a little headroom. Garnish each glass with a lemon or lime wheel and a sprig of mint if desired.

Serve immediately while fizzy. Leftover mixed soft drink will gradually lose carbonation; store any leftovers in a sealed bottle in the fridge and consume within 24–48 hours.