Chill the glass. Fill a tall glass halfway with ice.
Make the syrup. Add equal parts espresso or cold brew concentrate and lemon juice to the glass. Start with 2 to 3 tablespoons each in a 16-ounce glass.
Add 1 to 2 tablespoons of sugar or simple syrup.
Fill the glass with cold water and stir until well-combined.
Garnish with a lemon slice, if desired.
