Choose a corned beef brisket (usually 3 to 5 lb) with the spice packet included
Rinse the brisket and pat dry
Place the brisket in a large pot or Dutch oven
Add enough water to fully cover the brisket by at least 1 inch
Add the spice packet (and any included pickling spices)
Bring to a gentle boil over medium-high heat
Reduce heat to a low simmer
Skim foam if needed
Cover and simmer until tender (about 3 to 5 hours, depending on thickness)
Check doneness by piercing the thickest part with a fork (should go in easily)
Remove from heat
Let rest 10 to 20 minutes
Slice across the grain
Serve hot (optional: thicken cooking liquid for a quick sauce, or serve with cabbage and potatoes)