In an 8-quart or larger stockpot, combine 12 cups water, kosher salt, garlic, brown sugar, pink salt, and 3 tablespoons of pickling spice.
Bring the liquid to a boil, stirring until the sugar and salt dissolve. Cover the pot with a lid to make the liquid boil faster, checking on it every 5 minutes. Remove from the heat and let cool for 30 minutes.
Line a large bucket, tub, pot, the insert of a crockpot, cooler, or other similar vessel large enough to hold the brisket and brine snugly with a brining bag. Add the ice.
Pour the semi-cooled brining liquid on top (if it is still a little warm, the ice will melt and cool it down the rest of the way). Allow this mixture to sit and come to room temperature, about 1 hour.
Place the brisket into the fully cooled liquid (your patience is worth it for food safety!) and submerge so it’s covered in liquid. If the brisket floats, weigh it down with a heavy pot, bag of ice, or similar (we top ours with a stack of dinner plates that is about the circumference of the bucket; if you are using your crockpot, you can flip the lid upside down and use that).
Let the brisket brine for at least 5 days (or up to 7 days), ensuring that it stays below 40 degrees but above 32 degrees so that it does not freeze. You can place it in a refrigerator, in your garage, or outside if it's cold enough (just make sure it's not below freezing).
Cooking the Corned Beef (the day of):
When ready to cook, remove the brisket from the brine. Discard the brine. Rinse the brisket all over, then pat very dry. Trim off any excess fat with a paring knife, being careful not to cut away any of the meat itself.
Place the brined brisket in a large stockpot (6 quarts or larger). Add water until it is at least 1 inch above the brisket. Add garlic cloves, bay leaf, and 1 tablespoon pickling spice.
Bring the liquid to a boil over high heat. Reduce the heat, keeping the liquid at a low simmer. Cover the pot and let simmer for about 3 hours, until the brisket is fork tender and registers 180 degrees F to 190 degrees F on an instant read thermometer.
Turn the heat off and allow the meat to cool for 10 minutes. Use tongs to remove the corned beef to a cutting board. Remove 2 cups of the liquid from the pot. Strain the liquid into a bowl and set aside. Discard the remaining liquid from the stockpot. Rinse the pot.
Allow the corned beef to sit at room temperature for 20 minutes.
When you're ready to slice the corned beef, cut against the grain into 1/4" inch strips.
*Depending upon your brand of pickling spice, it can be a little heavy on the allspice. If you prefer, you can pick out a few of the allspice berries, or enjoy it as is.
*TO MAKE YOUR OWN PICKLING SPICE, stir together the following: 1/2 tablespoon each yellow mustard seed, multi-colored peppercorns, coriander seeds, allspice berries, red pepper flakes, and whole cloves; 2 whole cardamom pods; 1/2 cinnamon stick, crushed; 2 dried bay leaves, broken into pieces; and 1 teaspoon ground ginger.