Fill a cocktail shaker with ice then pour in lemon juice, gin and simple syrup. SHake well until very cold.
Pour the gin mixture into a highball glass filled with ice.
Top off with soda water and garnish with a lemon slice and cherry.
Notes
Don't worry if you don't have bottled simple syrup just bring equal amounts of granulated sugar and water to a boil until the sugar dissolves then leave to cool completely.
Although a lot of recipes use club soda I find that there is enough sweetness in the simple syrup. You can definitely use club soda or even tonic water if you prefer though.
To adjust the sweetness you can add more or less simple syrup.
A tall glass is best for this filled with ice.
Make sure to use fresh lemon juice please don't use bottled stuff I'll ruin your cocktail.
You should be able to buy Maraschino cherries in jars in most supermarkets.
If you feel like experimenting you can play with different flavored simple syrup (berry, citrus, or floral) or add in some herbs such as rosemary, thyme, or mint.
Why not add in some sliced strawberries or blueberries, it looks really impressive when serving it to guests.