Peel and chop the ripe mangoes, then blend them in a blender until smooth to make the mango puree.

In a mixing bowl, combine the mango puree, water, sugar, and lemon juice.

In a small saucepan, heat the mixture over medium heat until warm.

Sprinkle the agar-agar powder over the warm mixture and whisk continuously until fully dissolved.

Pour the mixture into a square mold or container and let it cool to room temperature.

Refrigerate for at least 4 hours or until set.

Once set, cut the jelly into cubes and serve chilled.