Fill a large roasting pan with water and bring it to a boil. Add the canning jars and lids to sterilize.
Pour water, cider vinegar, salt, and sugar in a large stock pot and bring to a boil. Once boiling, reduce heat to a simmer.
Remove hot jar from pan with tongs and place on towel lined counter top.
Place ½ tsp mustard seed, ½ tsp red pepper flakes, 2 cloves garlic, and 1 dill head in jar and pack with green beans until full.
Pour boiling vinegar solution over cucumbers, place lid on jar and seal with screw cap.
Cover pickled green beans with towels to prevent drafts until completely cool. Be sure all lids have sealed. Any that did not seal will need to be refrigerated
2 cups of beans should make 1 pint of dilly beans. Increase the measurements of the brine if making more than one pint of beans.
If you prefer to process the green beans in a hot water bath, transfer jars to canner, fill with hot water. Cover with canner lid and bring to a boil. Boil for 10 minutes. Carefully lift rack from canner and rest the rack on the rim. Remove hot jars from canner and place on towel lined counter. Cover with more towels to prevent cool air from hitting jars. Let cool completely. Store on shelf until ready to consume. Store any unopened or unsealed jars in refrigerator.