Prepare the cucumbers: Wash the cucumbers thoroughly and trim the ends. Cut them into slices, spears, or chips, depending on your preference.
Prepare the jars: Wash two medium glass jars and their lids with hot, soapy water. Rinse them well and allow them to dry.
Add the seasonings: Place the fresh dill, garlic, black peppercorns, mustard seeds, and red pepper flakes into the jars.
Pack the cucumbers: Add the cucumber slices or spears to the jars. Pack them firmly but avoid crushing the vegetables.
Make the brine: Add the vinegar, water, salt, and sugar to a small saucepan. Heat over medium heat, stirring until the salt and sugar dissolve. Do not change the vinegar-to-water ratio.
Pour in the brine: Carefully pour the warm brine into the jars until the cucumbers are completely covered.
Remove air bubbles: Tap the jars gently on the counter or use a clean utensil to release any trapped air bubbles.
Cool and refrigerate: Secure the lids and allow the jars to cool briefly. Place them in the refrigerator as soon as practical.
Chill before serving: Let the pickles rest for at least 24 hours before serving. The flavor will become stronger after 48 hours.
Store safely: Keep the jars refrigerated and use the pickles within approximately two weeks for the best quality. Discard them if they develop mold, an unpleasant smell, or an unusual texture. Refrigerator pickle storage times can vary depending on the recipe and ingredients.