When using unflavored gelatin, you need to sprinkle it into a liquid, either water or juice, mix it up and let it stand for a few minutes. This will rehydrate the gelatin, each granulate. This will help to ensure that the jello will be smooth and without clumps. Never boil the gelatin. If you heat it up on the stove, bring it to a low simmer only or pour the boiling water/juice into the bloomed gelatin, like you would with most store bought jello.