Finely chop the peppers, then add them to a large pot along with the sugar, vinegar, and salt.
Bring to a boil and boil for 10 minutes. The mixture should reduce and thicken slightly.
Slowly add in the liquid pectin and boil hard for 1 minute. Let cool for 5 minutes, then pour using a clean, sterilized funnel into prepared sterilized half-pint jars, leaving 1/4-inch headspace. Wipe rims of jars with a dampened clean paper towel. Apply and adjust prepared lids.
Process in a water bath canner for 5 minutes.* Remove (these tongs are helpful!) and let cool and sit undisturbed for 24 hours. Check for vacuum seals (you can reprocess any that didn't set. If your jelly didn't set, you can reprocess and add 1-3 ounces of more pectin to help.)
Notes
When cutting the jalapenos, I recommend wearing plastic or rubber gloves and not touching your face while handling or cutting hot peppers. Make sure to wash your hands thoroughly with soap and water before touching your face or eyes.*For 0-1000 feet sea level, process for 5 minutes. For 1,001 - 6,000 ft, process for 10 minutes. Above 6,000 ft, process for 15 minutes.