Combine the granulated sugar and 1/2 cup of water in a small saucepan. Bring to a gentle simmer over medium heat, stirring until the sugar has fully dissolved (about 3–4 minutes). Remove from heat and let cool to room temperature.

Prepare citrus

While the syrup cools, zest one orange to get the orange zest (about 1 tsp). Juice fresh oranges until you have 1 cup of orange juice, then strain to remove pulp if a clearer soda is desired. Add 1 tablespoon of fresh lemon juice to the orange juice and stir.

Combine flavor base

In a pitcher, combine the cooled simple syrup, the orange-lemon juice mixture, orange zest, 1 teaspoon vanilla extract, and 1 teaspoon citric acid if using. Stir until fully incorporated. If you want the classic bright color, add 1–2 drops of orange food coloring and stir. Taste and adjust sweetness or acidity: add more syrup for sweetness or up to another 1/2 teaspoon citric acid for tang.

Finish with carbonation

Slowly add 3 cups chilled carbonated water to the flavor base and stir gently once or twice to combine. Do not vigorously stir to preserve carbonation. Taste and adjust—if it's too sweet, add a splash more carbonated water; if too flat, use fresher chilled soda water.

Garnish & Serve

Fill glasses with ice cubes (about 1/2 cup ice per serving). Pour the prepared Fanta-style soda over the ice, dividing evenly. Garnish each glass with an orange slice and a mint sprig. Serve immediately chilled.