Step 1: Wash and prepare your cucumbers. Slice them into rounds or spears, about 1/4-inch thick. If desired, you can soak the sliced cucumbers in an ice bath for 15-30 minutes for extra crispness, then drain thoroughly.
Step 2: In a medium saucepan, combine the white vinegar, water, salt, and sugar. Heat over medium-high heat, stirring until the salt and sugar are fully dissolved. Bring to a boil, then remove from heat and let cool slightly.
Step 3: While the brine cools, pack your clean 1-quart jar (or multiple smaller jars) with the sliced cucumbers, garlic slices, dill seeds, mustard seeds, and red pepper flakes. Pack them in as tightly as possible without smashing.
Step 4: Carefully pour the slightly cooled brine over the cucumbers in the jar, ensuring all the cucumbers are submerged. If needed, gently press down on the cucumbers to keep them under the liquid.
Step 5: Seal the jar tightly with a lid. Allow the jar to cool completely to room temperature on the counter. Once cool, transfer the jar to the refrigerator. For best flavor, chill for at least 24 hours before serving. The pickles will continue to develop flavor over several days.