This isn't some artificially flavoured sugary fizzy drink.  It's made from ripe dark cherries and fresh squeezed lime juice. First, either use a cherry pitter - or just halve the cherries and pick out the pit.  This birdbeak knife is handy for things like this.  I use a couple handfulls of cherries per litre of water.  Then juice, or muddle, the cherries.  Since it's just a few cherries, I don't like to dirty up a juicing machine, so smashing with a muddler works easiest.  Then, add the juice of two limes to the smashed cherries.  Strain out the pulp and set aside the juice.

It does need just a little sugar to add some sweetness.  I use about half as much sugar as juice.  Add the sugar and juice to a bit of water.  Then top it off the mixture with seltzer.  Adjust the ratio of all the items to suit your taste, because, ya know, everybody likes things a little different.