Step 1: Prepare your peppers. Wearing gloves, finely dice all bell peppers, jalapeños, and serrano peppers (if using). A food processor can make this step quicker, pulsing until very fine but not pureed.
Step 2: Combine peppers and vinegar. In a large, heavy-bottomed pot, combine the finely diced peppers and apple cider vinegar. Bring the mixture to a boil over medium-high heat, then reduce heat and simmer for 10 minutes to soften the peppers.
Step 3: Add pectin. In a small bowl, whisk together the powdered pectin with 1/4 cup of the granulated sugar. Add this pectin-sugar mixture to the simmering pepper mixture in the pot, stirring well to dissolve completely. Bring the mixture back to a rolling boil that cannot be stirred down.
Step 4: Add remaining sugar. Once a rolling boil is achieved, add the remaining 4 3/4 cups of sugar all at once. Stir continuously until the sugar is completely dissolved.
Step 5: Boil to set. Bring the jelly mixture back to a vigorous rolling boil and boil for exactly 1 minute, stirring constantly to prevent scorching. Remove the pot from the heat.
Step 6: Skim and prepare jars. Carefully skim off any foam that has risen to the surface of the jelly. Ladle the hot jelly into sterilized canning jars, leaving 1/4 inch of headspace.
Step 7: Seal jars. Wipe the rims of the jars clean with a damp cloth, then center the lids on the jars and screw on the bands fingertip-tight.
Step 8: Process in water bath. Place the filled jars into a boiling water bath canner, ensuring they are fully submerged. Process for 10 minutes (adjusting for altitude if necessary).
Step 9: Cool and check seals. Carefully remove the jars from the canner and place them on a towel-lined counter, undisturbed, for 12-24 hours to cool and seal. Check seals; lids should be concave and not flex when pressed. Any unsealed jars should be refrigerated and consumed within 3 weeks. Store sealed jars in a cool, dark place for up to 1 year.