Prep the Zucchini: Wash the zucchini thoroughly under cold running water and pat them dry with a clean kitchen towel. Slice the zucchinis into thin rounds or spears, depending on your preference.
Prepare the Pickling Liquid: In a medium saucepan, combine white vinegar, water, granulated sugar, kosher salt, garlic cloves, black peppercorns, mustard seeds, and crushed red pepper flakes.