Prepare Jars: Examine all jars for cracks or chips and discard any damaged jars. Wash jars and rings in hot soapy water, rinse thoroughly, and keep jars hot in a 180°F oven or hot water until ready to fill.
Heat Lids: Place new canning lids in a small saucepan, cover with hot water at 180°F, and keep hot until ready to use. Do not boil lids as this may damage the sealing compound.
Prepare Canner: Fill your boiling water bath canner with enough water to cover jars by 1-2 inches when submerged. Begin heating water to a rolling boil while preparing jelly.
Extract Watermelon Juice: Place cubed watermelon and 1/2 cup water in a large pot and cook over medium heat for 15-20 minutes, crushing fruit with a potato masher until completely broken down and juicy.
Strain Juice: Pour cooked watermelon through a fine-mesh strainer or jelly bag, pressing gently to extract juice. Measure exactly 4 cups of strained juice, adding water if needed to reach full measurement.
Combine Ingredients: Return measured watermelon juice to pot and stir in powdered pectin until completely dissolved. Add bottled lemon juice and bring mixture to a full rolling boil over high heat, stirring constantly.
Add Sugar: Add all 3 cups of sugar at once to the boiling juice mixture, stirring constantly to dissolve completely. Return to a full rolling boil that cannot be stirred down and boil hard for exactly 1 minute.
Test for Doneness: Remove jelly from heat and quickly skim off any foam with a metal spoon. Test consistency by dropping a small amount on a cold plate - it should wrinkle when pushed with your finger.
Fill Jars: Using a ladle and wide-mouth funnel, fill hot jars with jelly, leaving exactly 1/4 inch headspace. Remove air bubbles by running a plastic tool around the inside of each jar and wipe jar rims clean with a damp paper towel.
Process in Water Bath: Center hot lids on jars and apply rings finger-tight only. Using jar lifter, place filled jars on canner rack and lower into boiling water, ensuring jars are covered by 1-2 inches of water. Process for exactly 10 minutes, adjusting for altitude: add 5 minutes for 1,001-6,000 feet, add 10 minutes for above 6,000 feet. Remove jars and cool undisturbed for 12-24 hours.
