When using a fruit puree to make gelatin, begin by softening the gelatin in a little bit of water. (If you are using a thin puree like watermelon, you can soften the gelatin in a little bit of the cold puree instead.) I usually use around 2 Tbsp. water for every Tbsp. of gelatin.
Make your fruit puree by placing the small fruits, or small pieces of larger fruits, into a blender container and blending until smooth. Fruits with lots of water like watermelon will make a thin, juice-like puree that is perfect for making a smooth gelatin. Other fruits, like berries, will make a thicker puree that you can slightly water down (with either water, coconut water, or juice) if you prefer a smoother gelatin. You can also strain out any seeds for fruits like strawberries or raspberries that have small seeds. You should try end up with around 2 cups of puree mixture (whether watered down or not.)
Pour around 1 cup of puree into a saucepan and heat it on low to medium heat. It doesn't need to boil, but should be hot enough to dissolve the softened gelatin.
Add the softened gelatin to the saucepan with the fruit puree and stir until the gelatin is dissolved.
Remove the saucepan from the heat, and pour in the rest of the fruit puree, stirring it until the puree is well incorporated into the gelatin mixture. You can also add in some honey or maple syrup to sweeten the mixture if you feel it needs it.
Pour the mixture into serving dishes and place in the fridge until the gelatin is fully set.
Making gelatin from juice
Soften the gelatin in around 1/4 cup of the juice.
Heat around 1 cup of the juice over medium heat in a saucepan. It doesn't need to come to a boil, but should be warm enough to dissolve the gelatin well.
Add the softened gelatin to the saucepan with the juice, and mix together well until the gelatin is completely dissolved.
Remove from the heat source and add in the remaining juice, stirring until well incorporated. You can also add honey or maple syrup to sweeten it more.
Pour the mixture into serving dishes, and put into the refrigerator until the gelatin sets.
Nutritional information based on using 2 cups of pineapple juice.