Bring 2 cups of white vinegar and 1-1/2 cups of white sugar to a boil along with 1 tablespoon of salt, 1 tsp. of celery seeds and 1 tsp. of red pepper flakes. Boil just until the sugar dissolves and remove it from the heat. Throw in 2 cups of ice cubes and let this mixture cool.
Thinly slice 4 cucumbers and 1 red onion. It is best to use a mandolin to do this.