Slice celery into ½-inch pieces, dice, or cut into batons.
Pack the celery into a wide-mouthed jar.
In a saucepan, combine vinegar, water, sugar, salt, peppercorns, and red pepper flakes. Heat until dissolved.
Pour the brine over the celery in the jar.
Add chopped dill, and gently stir.
Let the mixture cool to room temperature.
Seal the jar and refrigerate.
Enjoy immediately or let it sit overnight for stronger flavor.