Prepare Jalapenos: Wear gloves, remove stems, and thinly slice jalapeños.
Make Syrup: In a large pot, combine sugar, apple cider vinegar, garlic powder, turmeric, celery seed, and cayenne (if using). Bring to a boil, then reduce heat and simmer for 5 minutes.
Add Jalapenos: Stir jalapeños into the syrup. Simmer for 10-15 minutes.
Fill Jars: Use a slotted spoon to transfer jalapeños to jars, filling to ½ inch from the rim. Pour remaining syrup into jars.
Seal Jars: Wipe the jar rims, place lids, and tighten.
Process in Water Bath: Boil jars for 10 minutes, then remove and let cool. Store in a cool, dark place.