For each flavour of jelly, heat the juice and water (if using) in a pan over medium heat. Gradually sprinkle in and completely dissolve the gelatin, whisking after each addition. Pour the liquid into any container, like a glass or a mug. Place this in the fridge to fully set. This takes around 2-4 hours.
Once the jelly has fully set, cut it into little cubes. Then place the jelly cubes into champagne glasses until the little cubes reach the top of the glasses.
Pour champagne on top of the jellies, and enjoy with a spoon!