Rinse the beans under cool water and drain well. Trim the stems from the beans.
Place pepper flakes, mustard seeds, and garlic in the jar.
Lay the jar on its side, place one sprig of dill inside then stack the beans until the jar is half full, layering dill sprigs evenly with the beans. Pack the jar as full as possible, a full pound should fit in a quart sized jar.
Heat the vinegar, water and salt in a small saucepan and bring to a boil. Pour the brine over the green beans, filling the jar to within ½ inch of the top.
Gently tap the jar against your countertop to release any bubbles, top off with more brine if needed. Screw the lids on tight.
Allow the jars to cool to room temperature before storing in the refrigerator. Wait at least 48 hours before eating as they will continue to improve in flavor the longer they sit.
The dilly beans will last for up to 2 months in your refrigerator. If you process the jars they can be stored at room temperature until opened.