In a heatproof bowl, carefully pour the boiling water. Add the sugar-free gelatin mix and whisk vigorously for about 2 minutes, or until the gelatin is completely dissolved. Ensure there are no granules left at the bottom.
Stir in the cold water and any optional fruit juice. Whisk again to combine thoroughly. This step helps to cool the mixture slightly and prepare it for setting.
Pour the liquid mixture into individual serving bowls, ramekins, or small molds.
Place the bowls or molds in the refrigerator and chill for at least 2-4 hours, or until the gelatin is firm and set.
Once set, you can serve the gelatin as is, or garnish with fresh berries or a small amount of sugar-free whipped topping if desired.