If using high fructose corn syrup, measure and place directly into a small mixing cup. If using granulated sugar, combine the sugar with 1 tablespoon hot water and stir until fully dissolved to make a simple syrup. Allow to cool slightly.

Combine acids and citrus

In the same mixing cup, add the fresh lemon juice (1/2 tsp), fresh lime juice (1/4 tsp) and citric acid (1/4 tsp). Stir to combine — this adds the bright tang characteristic of the beverage.

Add flavor concentrates

Add the orange and lemon-lime flavorings (1/8 tsp each) to the syrup/acid mix. If using powdered or concentrated flavor, ensure it is fully incorporated. If you are adding caffeine, dissolve the measured caffeine (30 mg) into a small amount of warm water first and stir into the mix.

Adjust color

Add 1 drop yellow food coloring and, if you want a neon green tint, add 1 drop blue food coloring. Stir gently; a single drop of each is enough for a 12 fl oz serving. Adjust by adding another tiny drop if necessary.

Assemble Beverage

Fill a 16 fl oz glass with ice (about 1 cup). Pour the prepared sweetener/flavor/color/citric mix over the ice.

Slowly add 12 fl oz chilled carbonated water (club soda or seltzer) to the glass to preserve effervescence. Stir gently with a spoon or swizzle stick 2–3 times to distribute the flavor without flattening the soda.

Taste and adjust: if the drink is not sweet enough, add up to 1/2 tablespoon additional sweetener (dissolved first). If too sweet, add a splash more carbonated water and a few drops more citric acid or lemon juice for brightness. Adjust color with a single additional drop of coloring if desired.

Garnish: add a lime wedge or lemon wheel on the rim if desired, and serve immediately while fizzy.