Prepare the Cucumbers: Wash the cucumbers thoroughly. Slice them into spears or keep them whole, depending on your preference.
Make the Brine: In a saucepan, combine vinegar, water, and kosher salt. Heat over medium heat, stirring until the salt dissolves. Remove from heat and let cool to room temperature.
Pack the Jars: Pack the cucumbers, garlic cloves, and dill into your glass jars. Add mustard seeds, peppercorns, and red pepper flakes if desired.
Add the Brine: Pour the cooled brine over the cucumbers in the jars, making sure they are fully submerged. Leave about 1 inch of space at the top.
Seal and Refrigerate: Seal the jars tightly and refrigerate. Let them sit for 24-48 hours for the flavors to develop. The longer they sit, the more flavorful they become.