It's best to use cooked corned beef for this slow cooker recipe. Cooking the corned beef in advance ensures it becomes tender and allows the flavors to develop fully during the slow cooking process. You can use leftover corned beef from a previous meal or cook it specifically for this recipe. Or, you can cook the corned beef in the slow cooker and add in the other ingredients once the corned beef is fully cooked. Simply toss the corned beef in the slow cooker with 12-16oz of liquid.