Chop the watermelon and remove the rinds and seeds.

Add chopped watermelon in a blender and purée until smooth. Depending on how much jelly you are making, you might have to do this in batches.

Place a fine mesh strainer over a bowl and pour the puréed watermelon through it to strain out the pulp.

Measure out 4 cups of of watermelon juice. You can reserve any leftover juice to enjoy later. Just keep it chilled in the refrigerator.

Add the watermelon juice, lemon juice and sugar into a heavy pot and bring to a rolling boil. Stir until sugar is dissolved and continue cooking.

Stir in the pectin and continue cooking until the jelly reaches 225°F. Skim off and discard any foam.

Carefully ladle the hot watermelon jelly into prepared jars, leaving ¼-inch headspace. Wipe down the rims and sides of each jar with a clean kitchen towel. Screw on the lids.

You can let the jelly set in the refrigerator or process the jars in boiling water or a canner for 10 minutes. You can find full directions on hot water bath canning here. Any unsealed jars should be refrigerated and used within 1 month.