Soak Brussels sprouts in a large bowl filled with lightly salted water for about 15 minutes. Drain well.

Sterilize the jars and lids in boiling water for at least 5 minutes.

Divide drained Brussels sprouts evenly between jars, filling the jars about 3/4-inch from the top.

Place 1 garlic clove and 1/4 teaspoon red pepper flakes in each jar.

Bring vinegar, water, and pickling salt to a boil in a large pot over medium-high heat until salt is dissolved, about 5 minutes.

Pour vinegar mixture into the jars, filling the jars to within 1/4 inch of the top.

Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles.

Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids, and screw on rings.

Place a rack in the bottom of a large stockpot, and fill stockpot halfway with water.

Bring to a boil over high heat, then carefully lower the jars into the pot using a holder. Leave a 2-inch space between the jars. Pour in more boiling water if necessary, until the water level is at least 1 inch above the tops of the jars. Bring the water to a full boil, cover the pot, and process for 10 minutes.

Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, until cool.

Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark area, and wait at least 3 weeks before opening.