Red syrups of any flavors you likebut usually Indonesians use either Ros or Cocopandan flavor (shown on picture). Available online and/or at Asian stores.
Cold club soda
Sweetened condensed milknot evaporated milk.
Instructions
Pour as much sweetened condensed milk into serving cup as you like (more for creamier and sweeter), follows with the syrup, the measurement is depending on how much milk you added earlier.
Add in some ice cubes, and just before serving, pour in cold club soda.