Step 1: Sprinkle the gelatin powder evenly over the 1 cup of cold water in a heat-proof bowl. Let it sit undisturbed for 5-10 minutes to "bloom" and soften. The granules should swell and become translucent.
Step 2: Place the bowl of bloomed gelatin over a saucepan of simmering water (or microwave in 20-second intervals) and stir gently until the gelatin is completely dissolved and the liquid is clear. Do not boil.
Step 3: In a separate large bowl, combine the 3 cups of hot water (or hot fruit juice) and granulated sugar (if using). Stir until the sugar is fully dissolved.
Step 4: Slowly pour the dissolved gelatin mixture into the hot liquid base while whisking continuously for 1-2 minutes to ensure thorough incorporation.
Step 5: Stir in the 1 cup of cold fruit juice or other cold liquid. This helps to cool the mixture down slightly.
Step 6: Allow the mixture to cool to room temperature. Once cooled, gently fold in the chopped fresh or canned fruit. If you want the fruit suspended evenly, wait until the gelatin mixture is slightly thickened but not fully set before adding.
Step 7: Pour the entire mixture into your desired mold (a 9x13 inch pan, an intricate gelatin mold, or individual serving dishes).
Step 8: Refrigerate for a minimum of 4 hours, or preferably overnight, until the gelatin is completely set and beautifully wobbly. If unmolding, briefly dip the bottom of the mold in warm water before inverting onto a serving plate. Serve chilled.