1. Combine the vinegar, water, salt, mustard seeds, and chili flakes in a small saucepan and bring to a boil. Once boiling remove from heat and add the dill.
2. Prep the cucumber while the mixture comes to a boil. For an English cucumber, slice the cucumber into ¼-inch rounds. For a slicing cucumber, peel, slice in half lengthwise, and use a spoon to scoop out the seeds before cutting into ¼-inch slices. Pack cucumber slices and garlic into a pint jar with a tight-fitting lid.
3. Pour the hot liquid over the cucumbers. The cucumbers should be just covered; discard any extra liquid you may have or add more vinegar and water (in a 2:1 ratio) as needed to cover. Cap tightly and swirl the cucumbers gently before transferring to the refrigerator.
4. Quickles are ready once they have cooled completely, about an hour, but they taste best if allowed to chill for 5-24 hours.