Make the simple syrup. In a small pot, combine 1/2 cup sugar and 1/2 cup water.
Warm over low heat, stirring until the sugar dissolves. Let it cool fully. For a lighter sweetness, use 1/3 cup sugar instead.
Juice the lemons. Roll 3–4 lemons on the counter to loosen the juice.
Cut and squeeze until you have about 1/2 cup fresh lemon juice. Strain out seeds and most pulp for a smooth sip.
Chill your glassware. Pop your serving glasses in the freezer for 5–10 minutes. Cold glasses keep the bubbles lively and the drink crisp.
Build the base. In a pitcher, combine the lemon juice with 1/4 to 1/3 cup simple syrup, 1/2 cup cold still water, and a tiny pinch of salt.
Stir and taste. You want bright, not harsh, with a gentle sweetness.
Add ice to glasses. Fill each chilled glass halfway with ice. If you like it extra fizzy, use larger cubes so they melt more slowly.
Top with bubbles. Pour the lemon base into each glass, filling about halfway.
Top with very cold sparkling water. Gently stir once to combine without knocking out the carbonation.
Garnish and finish. Add a lemon wheel, a sprig of mint or basil, and a few berries if you like. For a complex edge, add 2–3 drops of bitters to each glass.
Taste and adjust sweetness or bubbles on the spot.
Serve immediately. This drink shines when it’s icy cold and full of fizz. Keep extra sparkling water nearby for top-ups.