Boil the potatoes in salted water until tender, about 15-20 minutes.
Prepare the pickling brine by combining vinegar, water, garlic, dill, salt, sugar, peppercorns, and optional red pepper flakes in a saucepan and bringing to a gentle boil.
Combine the cooled potatoes with the brine in a clean jar.
Seal the jar tightly and refrigerate. Let marinate for at least 24 hours for best flavor.
Notes
Store in the refrigerator for up to 2-3 weeks. Flavors improve over time.