Please read About Prickly Pears, About Making Jelly, and How to Process. Wear gloves when handling prickly pears.
Combine in a large heavy saucepan:
Bring to a boil, stirring frequently, then reduce the heat to simmer for 5 minutes, mashing the fruit into a pulp with a potato masher. Remove from the heat and extract the juice. While the juice drains, prepare a water-bath canner, gather all your canning equipment, put a couple of saucers in the freezer, and have ready 5 sterilized hot half-pint jars.
Measure the strained juice: You should have about 3 cups; add water if necessary. Pour the juice into a large heavy saucepan. Add:
Stir and bring to a boil over high heat. Whisk together in a medium bowl and stir into the boiling juice:
Stirring constantly, return to a boil and boil according to the package directions. Use the wrinkle test to make sure the mixture has reached the jelling point. Pour into jars, leaving ¼ inch headspace. Wipe the rims. Place lids on jars and screw on rings until fingertip-tight. Process for 5 minutes. Let cool completely and store as directed.