Bring the vinegar, sugar and salt to a boil in a small saucepan until the sugar and salt are dissolved.
Add the mustard seed, chili flakes and peppercorns and simmer an additional 5-7 minutes.
Cut the celery stalks into 4-5" pieces and pack into the jar along with the garlic.
Pour the vinegar brine over the contents of the jar, make sure all of the pieces are submerged in the brine. Cover the jar and allow to cool completely before refrigerating.
As the spicy celery stalks cool, you may have to shake the jar to distribute the spices and brine.