Certified GKH Gardening Expert
If you plan on harvesting it as a head, you will wait until it has grown to a good size (think roughly what you would see in the store) . A gentle squeeze will tell you if it is firm or loose. A well developed head will be firm. Sometimes in hot weather, the head will be looser, but it can still be harvested.
To harvest, simply cut the head from the base of the plant.