Harvest cabbage heads before the first hard frost. They can easily survive soft frosts, which may even sweeten their flavor.
Wait until the cabbage heads become solid and fully formed before harvesting. Pick the heads before they crack open.
Snip off the outer leaves from the plant with a pair of clean, sharp garden scissors.
Grip the head, and twist it a quarter-turn to sever the roots and release it from the plant. If you want further harvest, cut the head from the plant with a sharp knife, keeping as much of the stem intact as possible. A smaller head can develop at the base of the harvested head if the stem is left.