In a medium bowl, combine the white vinegar, water, sugar, salt, mustard seeds, dill seeds, garlic powder, red pepper flakes (if using), black peppercorns, and coriander seeds. Stir until the sugar and salt are dissolved.
Place the sliced cucumbers in a clean, quart-sized jar or container.
Pour the vinegar mixture over the cucumbers, ensuring they are fully submerged. If needed, press down on the cucumbers to remove air bubbles.
Seal the jar tightly and refrigerate for at least 2 hours before serving. For best flavor, let them chill for 24 hours.
The pickles will keep in the refrigerator for up to 2 weeks.