While the jalapeños are cooking in the syrup, get your water-bath canner ready and bring the water up to a boil.
Prepare your jars, making sure they are completely clean and free of cracks and defects. Prepare your canning lids.
Heat the canning jars by placing them in the water to warm up.
Once the relish is ready to be packed, remove the jars and dump all the water back into the canner.
Place the hot jars on a clean kitchen towel and put a canning funnel on top of the first jar to help keep things tidy. Use a slotted spoon to scoop out jalapeños and divide the peppers up as evenly as possible into the jars.
Ladle the hot syrup into each jar leaving a 1/4-inch headspace.
Debubble the jars. Wipe the rims of the jars with a damp paper towel and again with a dry paper towel to make sure the rims are clean and able to seal properly.
Place a canning lid on each jar and screw the ring on until it is finger-tight. Then lower the jars into the water bath, cover with a lid and bring the water to a roiling boil.
Once the water is boiling, set a timer for 15 minutes if using pint jars, or 10 minutes if using smaller jars.
When the jars have finished being in the boiling water bath, remove each jar carefully and place on a kitchen towel on the counter. Leave them alone for 12 hours without touching.
After 12 hours, remove the rings and check the seal of each jar. Label and date your jars.