Bring water, vinegar, salt, and sugar to a boil in a small saucepan. (The sugar and salt will dissolve.)
Remove the saucepan from the burner, stir, and allow the liquid to completely cool.
Wash a large handful of green beans, trim the ends so they will fit in the jar, and set aside.
Add garlic, dill, coriander, and red pepper flakes to the bottom of the mason jar.
Add green beans to the jar.
Pour cooled pickling liquid into the jar leaving 1/4" space near the top.
Secure the jar's lid.
Refrigerate for 24 hours.