Prepare the Cucumbers: Slice cucumbers into spears or rounds. Soak in ice water for 15 minutes for extra crispiness (optional).
Make the Brine: In a saucepan, combine vinegar, water, sugar, and salt. Bring to a simmer, stirring until dissolved. Add garlic, mustard seeds, black peppercorns, and red pepper flakes (if using). Simmer for 5 minutes.
Pack the Jars: Sterilize jars by boiling them in hot water. Pack cucumbers tightly into jars and add a sprig of dill.
Pour the Brine: Carefully pour the hot brine over the cucumbers until fully submerged. Seal jars with lids.
Cool and Refrigerate: Let the jars cool to room temperature, then refrigerate for at least 4 hours. For best results, refrigerate for 24 hours before enjoying.