Cut a ripe watermelon in half and scoop out the flesh. Blend until smooth, then strain to get about 4 cups of watermelon juice.

Pour the watermelon juice into a pot and bring to a boil over medium-high heat, cooking for 15-30 minutes until concentrated.

In a bowl, mix 1 cup of sugar with low-sugar pectin. Whisk this mixture into the boiling juice.

Allow to return to a rolling boil, stirring for 1-2 minutes. Squeeze in fresh lemon juice and boil until pectin activates.

Prepare sterilized jars and fill with hot jelly, leaving ½ inch of headspace. Clean rims and seal.

Process jars in a water bath canner for 5-10 minutes, then let cool on a towel for at least 12 hours.