While we used coffee concentrate, which requires you to cut the flavor naturally with milk or water, you could indeed use brewed coffee. If you were to use brewed coffee, try a 2:1 ratio of cold coffee to sparkling water. Add fizz and adjust the ratios as necessary.
When it comes to sparkling water flavors, some swear by Topo Chico lime, but if you're outside of Texas, this might be hard to find. Lime LaCroix didn't have the right zing to it, and some felt the citrus overpowered the drink. We recommend starting with plain and branching out from there.
Want to learn how to make your own cold brew? You got it, dude!