Make the brine. In a small saucepan set over medium-high heat, bring the vinegar, water, salt, sugar, pickling spices, and red pepper flakes (if using) to a simmer, stirring occasionally until the sugar and salt are dissolved.
½ cup apple cider vinegar, ½ cup water, ½ tbsp kosher salt, 1 tsp granulated sugar, 1 tsp pickling spices, pinch crushed red pepper flakes