Combine the sugar and water in a small saucepan over medium-low heat. Stir occasionally until the sugar fully dissolves, which usually takes just a couple of minutes. Avoid letting the mixture boil, since that can burn the sugar or evaporate too much liquid.
While the syrup cools slightly, juice enough fresh lemons to measure one full cup. This typically takes around four to six medium lemons depending on their size and juiciness. Fresh juice makes a noticeable difference in flavor compared to bottled versions.
Pour the cooled simple syrup and fresh lemon juice into your pitcher. Add four to five cups of cold water, adjusting based on how strong or mild you like your lemonade. Stir everything together until fully combined.
Place the pitcher in the refrigerator and let it chill for two to three hours. This resting time allows the flavors to settle and get properly cold before serving. Skipping this step can leave your drink tasting a bit too sharp.
Right before serving, fill each glass about two-thirds full with your chilled lemonade base. Top the remaining third off with cold sparkling water so it stays bubbly and fresh. Garnish with a lemon or lime slice for a beautiful finishing touch.