If you're using a lemon for the lemon juice, juice the lemon
Fill a highball glass or ice tea glass with ice
Pour lemon juice over the ice
Pour in the simple syrup
Fill the glass to the top with sparkling water
You want at least 40 ml of lemon juice to make this recipe work. 1 very large and juicy lemon or 2 medium lemons gives you about 50 ml of lemon juice.
Alternatively you can pick up lemon juice from the supermarket - I use the Santa Cruz brand lemon juice as it keeps on the shelf when unopened almost indefinitely, and after opening, stores in the refrigerator for weeks. It is also cheaper than buying the lemons and juicing them yourself.