bowl of strawberry and bananas jello topped with marshmellos
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Preparing JELL-O involves mixing the supplied gelatin with boiling hot water, adding the appropriate amount of cold water and placing the end result in the refrigerator to set for a few hours. While it sounds straight-forward enough, there are a lot of ways to mess up JELL-O, especially when additional ingredients like fruit are added to the recipe. If your JELL-O mix will not set, you can try to salvage it. If it can't be salvaged, however, you'll need to start the process over again. But by following some basic tips at the outset -- and by relying on some remedies after the JELL-O has been made -- you should be able to make a final product that sets properly.

Dissolve gelatin in boiling water before the cold water is added to the mix when preparing it from scratch. If the gelatin is not completely dissolved before cold water is added, it will not set properly.

Place the JELL-O in the refrigerator and allow it to set for at least six hours. JELL-O poured into thick molds and JELL-O containing fruits and vegetables take longer to set.

Cover the JELL-O pan with plastic wrap if you're leaving it to set overnight. This will prevent the JELL-O from hardening and allow it to set correctly.

Remove all fruits that can prevent the JELL-O from setting. Fruits such as guava, figs, ginger, papaya, pineapple and kiwi all contain an enzyme which breaks down the gelatin and prevents it from setting properly.

Pick out fruits and vegetables from the JELL-O until it has begun to thicken. The JELL-O should remain in the refrigerator for at least 1 1/2 hours before removing fruits and vegetables in this way. Make sure canned fruit and vegetables are well-drained. Never use fresh or frozen fruits that contain the enzyme that breaks down the gelatin. Re-refrigerate the JELL-O for an additional four hours.

Add only 1 1/2 times the amount of salt, vinegar and other wet ingredients the recipe calls for when preparing twice the amount of JELL-O. When doubling the recipe, add twice the amount of gelatin, liquid and fruit but don't ration other ingredients, such as lemon juice or salt the same way.