In a large pot, bring cider vinegar, sugar, garlic powder, cayenne and celery seed to a boil.
Reduce heat and simmer for 5 minutes.
Add the jalapeno pepper slices (and red pepper, if desired) and simmer for 4 minutes.
Use a slotted spoon to transfer the peppers into clean, sterile canning jars, packing the jalapenos semi-firmly within ¼" of the rim of the jar.
Turn heat up for the remaining syrup and bring to a full rolling boil.
Boil for 5 minutes.
Use a ladle to pour the boiling syrup into the jars over the jalapeno slices.
Insert a knife into each of the jars two or three times to release and trapped pockets of air.
Add more syrup if necessary.
Wipe the rims of the jars with a clean, damp paper towel, then screw on the two-part canning lids.
If you do not want to properly can these, simply cool jars at room temperature for an hour and then store them in a sealed container in the fridge for up to 3 months.
Use proper canning procedures to store at room temperature for longer periods of time.
To can, place jars in a canner and cover with water by 2". Bring the water to a full rolling boil. When it reaches a boil, set a timer for 10 minutes for half pint jars or 15 miniutes for pint jars. When the timer goes off, use canning tongs to transfer the jars to a cooling rack or kitchen towel. Leave them to cool, undisturbed, for 24 hours. When fully cooled, wipe them with a clean damp washcloth. Double check that they've sealed properly by pushing down on the center of the lid. It will not move if it is sealed.