In a small saucepan, combine the vinegar, salt, and sugar. Warm over medium heat, stirring just until the salt and sugar dissolve. There’s no need to boil — gentle heating is enough.
Remove from heat and stir in the cold water. Set the brine aside to cool completely, or refrigerate briefly until chilled.
Pack the cucumbers snugly into a clean 1-quart jar. Tuck the garlic, coriander seeds, mustard seeds, red pepper flakes, and dill in among the cucumbers.
Once the brine is cool, pour it over the cucumbers until fully submerged. If needed, add a splash of cold water or vinegar to ensure everything is covered.
Cover the jar and refrigerate for at least 24 hours before serving. The flavor will deepen after 48 hours.
Store in the refrigerator for up to one month. Ensure that pickles stay immersed in the brine.