Begin by selecting a ripe watermelon and cutting it into chunks. Blend until smooth and strain through a fine-mesh sieve into a bowl. Let juice sit at room temperature while gathering other ingredients.

In a mixing bowl, combine glutinous rice flour, granulated sugar, and fresh watermelon juice. Whisk until completely smooth with no lumps.

Transfer the bowl to the microwave. Cook in short intervals, stirring after each minute, until the mixture is glossy and translucent (about 4 to 5 minutes).

Take the mixture out and dust a plate with cornstarch. Transfer the warm mochi mixture onto the plate and allow it to cool. Dust hands with cornstarch to shape into small balls or discs.

Let the mochi cool completely before serving. Dust with additional cornstarch if desired to prevent sticking.