Start by removing the flesh of the watermelon and blending it to create juice. Ensure there are no seeds in the juice.
Pour the watermelon juice through a sieve to extract the fresh juice, leaving behind any pulp or solids.
In a large pot, combine the watermelon juice, agar agar powder, sugar / natural sweetener (if using), and water. Stir to combine all the ingredients thoroughly.
Place the pot on the stove over medium heat and bring the mixture to a boil, stirring continuously to prevent lumps from forming.
Once the mixture reaches a boil, remove it from the heat and pour it into the empty watermelon shell. This adds a fun and unique touch to the presentation of your jelly.
Refrigerate the jelly for 3 to 4 hours, allowing it to set completely.
Once set, slice the jelly into watermelon-shaped pieces, resembling real watermelon slices.