Sterilize a quart-sized jar by washing it with hot, soapy water or running it through the dishwasher.
Wash the cucumbers thoroughly. You can leave them whole if they’re small or slice them into spears or rounds for easier pickling.
Place the cucumbers in the jar along with the fresh dill sprigs, minced garlic, black peppercorns, red pepper flakes (if using), celery seed, and mustard seeds.
In a saucepan, combine the water, distilled white vinegar, and salt. Bring to a simmer and stir until the salt is completely dissolved.
Carefully pour the hot brine over the cucumbers and spices in the jar, ensuring everything is fully submerged. Top off with additional water if needed.
Allow the jar to cool to room temperature, then cover it with a lid and refrigerate. Let the pickles sit in the refrigerator for at least 24 hours to develop flavor—longer is better, with optimal taste after 3-4 days.