Bring the water to a full boil. Remove from heat, add the black tea bags, and steep for 5 minutes to create a strong tea base. Remove the tea bags without squeezing.
While the tea is still hot, stir in ⅓ cup sugar until fully dissolved. Taste and add more sugar gradually, up to ½ cup total only if needed. This step is essential — Southern sweet tea is always sweetened hot.
Let the sweetened tea cool to room temperature. If desired, add a small amount of cold water or ice to adjust strength, keeping the tea smooth and balanced rather than heavy.
Refrigerate for 1–2 hours until fully cold and refreshing.
Fill glasses with ice, pour over the chilled sweet tea, and garnish with a lemon slice if desired.