Wash the cucumbers thoroughly. Trim off the blossom end of each cucumber (this helps prevent sogginess). You can leave them whole, slice them into spears, or cut them into rounds, depending on your preference.

In a clean quart-sized glass jar or similar container, place the smashed garlic cloves, dill fronds, peppercorns, mustard seeds (if using), and red pepper flakes (if using).

Pack the prepared cucumbers snugly into the jar, on top of the spices and herbs.

In a small saucepan, combine the water, vinegar, kosher salt, and sugar. Bring the mixture to a boil over medium-high heat, stirring until the salt and sugar are dissolved.

Carefully pour the hot brine over the cucumbers in the jar, making sure they are fully submerged. Leave about 1/2 inch of headspace at the top.

Let the jar cool to room temperature on the counter. Once cooled, screw on the lid tightly and refrigerate. The pickles will be ready to eat after 24-48 hours, but the flavor will continue to develop over the next few days.