Prepare the watermelon juice
First, scoop out the pulp of the Watermelon and transfer it to a mixer jar. Blend into a smooth paste, then strain it using a sieve to extract fresh watermelon juice.

In a pan, add the watermelon juice along with a cinnamon stick, star anise, and sugar. Cook on medium heat for about 10–15 minutes until the juice slightly thickens.

Add the chopped watermelon rind and cook for 15–20 minutes until it becomes soft and slightly translucent.
Thicken the mixture
Simmer on low to medium heat, stirring occasionally. The mixture will gradually thicken (about 20–30 minutes).

Once it reaches a thick, glossy, jam-like consistency, turn off the heat. Remove the cinnamon stick and star anise. Let it cool completely.

After cooling, blend the mixture into a smooth paste.
Transfer it back into the pan and cook again on low to medium heat for 15–20 minutes, stirring well until it becomes thick.

Allow it to cool completely, then transfer to a clean, dry jar. Store in the refrigerator.