Step 1: Prepare Your Cucumbers. Wash cucumbers thoroughly. Slice them into ¼-inch rounds or cut into spears (quarters lengthwise).
Step 2: Pack the Jars. Divide the garlic, dill sprigs, mustard seeds, and red pepper flakes (if using) evenly between two clean, pint-sized glass jars. Tightly pack the cucumber slices or spears into the jars.
Step 3: Prepare the Brine. In a medium saucepan, combine the distilled white vinegar, water, salt, and sugar. Bring the mixture to a rolling boil over medium-high heat, stirring until the salt and sugar are completely dissolved.
Step 4: Pour the Brine. Carefully pour the hot brine over the packed cucumbers in the jars, ensuring all cucumbers are completely submerged. Leave about ½ inch of headspace from the rim of the jar.
Step 5: Seal and Cool. Tap the jars gently on the counter to release any trapped air bubbles. If needed, add more brine to maintain the ½-inch headspace. Place lids on the jars. Allow the jars to cool to room temperature on your counter (about 1-2 hours).
Step 6: Chill and Enjoy! Once cooled, transfer the jars to the refrigerator. For the best flavor development, let the pickled cucumbers chill for at least 24 hours before enjoying. They will be delicious after a few hours, but the flavor intensifies overnight. These refrigerator pickles will keep well for up to 3-4 weeks in the fridge.