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.