3tablespoonsagar agar flakes(or 3 teaspoons agar powder)
Grease a 9-inch square pan (or any size/shape pan that you like) with cooking spray and line it with parchment paper for easier jello removal later.
Pour the fruit juice into a small saucepan, and add in the agar agar flakes. Bring the juice to a boil, and whisk constantly to help the agar flakes dissolve. Once the juice is boiling the flakes have completely dissolved, remove the mixture from the heat.
Pour the liquid into the lined dish, then transfer the dish to your refrigerator to set. It will take up to 2 hours to cool completely. Slice into squares and serve chilled.