Combine canning salt, water, and vinegar in a large saucepan. Bring to boil; reduce heat and simmer 10 minutes (180 degrees).
Meanwhile, pack garlic in to 4 sterilized pint jars (about 8 ounces per jar) leaving 1/2-inch of headspace. Add 1/4 teaspoon red pepper flakes and 1 head of dill to each jar.
Using a ladle, divide hot pickling liquid between the 4 jars, leaving 1/2-inch of headspace. Remove air bubbles, clean jar rims, center lids on jars, and adjust band to fingertip-tight.
Chill in the refrigerator until pickled as desired (I recommend at least 3 weeks in the refrigerator). Store in refrigerator for up to 4 months (see notes) or seal jars following the instructions below (the pickled garlic must still be refrigerated; it will not be shelf-stable).