Remove the stems and seeds from the jalapeños. For a spicier jelly, leave some of the white pith attached.
Blend the jalapeños and 1 cup of the vinegar in a food processor until smooth.
In a large pot, mix the jalapeño puree, the additional vinegar, and pectin. Over high heat, bring to a full rolling boil that cannot be stirred down, stirring constantly for at least 1 minute.
Add the sugar and stir until it is fully dissolved. Bring to a boil and cook for 1-3 minutes, stirring constantly. If you own a kitchen thermometer, cook until the mixture reaches a temperature of 220°F.
Turn off the heat. Carefully ladle the jelly mixture into the hot jars, leaving enough room for the lid and jar rings.
Cover with lids and allow to set at room temperature, undisturbed, for 24 hours.