Simple syrup is equal parts sugar and water, heated until dissolved, then cooled.
Always use freshly squeezed lemon juice for the best flavor. One medium lemon yields about 1 oz (30 ml) of juice.
A Collins glass is the traditional choice, but any tall glass works.
Old Tom gin is the traditional spirit for a Tom Collins, with a slightly sweeter profile. London Dry gin works just as well for a crisper, more juniper-forward version.
Stir gently after adding the soda water to keep the fizz alive.
For a fruitier version, try adding a splash of homemade sweet and sour mix in place of the simple syrup and lemon juice.