The way we made it at the place I worked was to puree fruit, then reduce it a bit over heat (no added sugar), then put like 2tbsp or so over ice and topped off with the carbonated water.

So it’ll only really work well with flavors that don’t degrade in heat, but you may also have luck with just straight pureed fruits like watermelon, not super sure how that’d turn out.

You can also use fruit syrups that you buy or make yourself, tons of recipes out there.