Measure the strained prickly pear liquid and any pulp. Return to the large pot you originally cooked the chunks of prickly pears in.
For every cup of prickly pear liquid, add 3/4 cup granulated sugar and 1 teaspoon lemon juice. Also add the peel (including the white, pithy part) of half a lemon for every 2 cups of prickly pear liquid. The lemon peel provides pectin so that you get a good gel. Be sure to use only organically grown lemons - you don't want pesticides in your prickly pear jelly!
Cook over high heat, stirring often, until it reaches the gel point.