In a large pot, combine the water, kosher salt, curing salt, and brown sugar, heating gently and stirring until everything dissolves
Remove the pot from the heat and add the black pepper, mustard seeds, coriander seeds, juniper berries, bay leaves, apple cider vinegar, and the beer or apple juice
Let the brine cool completely at room temperature, then place it in the refrigerator until the brine is cold
Place the beef brisket in a large ziplock bag or glass bowl and pour the cold brine over it
Make sure the meat is fully submerged and seal tightly
Refrigerate for 5-10 days, flipping the brisket daily to ensure even curing