Rinse the corned beef under cold water
Place the corned beef in the slow cooker
Add the spice packet if included
Add enough water, broth, or a mix to cover about halfway
Add optional vegetables like onion, carrots, and potatoes around the beef
Cover and cook on low for 8 to 10 hours
Cook on high for 4 to 5 hours if needed
Check that the beef is fork-tender before removing
Transfer the corned beef to a cutting board
Let it rest for 10 to 15 minutes
Slice against the grain
Serve with the cooked vegetables and cooking liquid if desired