This is one of my favourite non-alcoholic drinks that I have a few times a week, but due to its bitter style, it's not to everyone's taste.
To make it how it's supposed to be made is very simple:
Take a tall glass and fill with ice.
Add a splash of Angostura bitters.
Add a splash of lime juice.
Fill with tonic water.
Top off with a lime slice if you want.
Between the bitters, lime and quinine, this is already a very tart drink. Even so, I personally take it further in that direction, and usually go a little heavy with the lime and bitters, and I skip the fruit on top.