Wait to harvest until some of the heads are formed and are about the size you'd find in the supermarket. Different plants grow at different rates, so don't expect to harvest all your lettuce in one day.
Bring a bucket or basket to the garden site. For easier carrying, use a bucket or basket with a handle.
Put on leather gloves for protect as you work with the knife. Bend down, grasp the lettuce head and lift it up from the ground and cut or pull away the loose foliage attached to the base of the lettuce head.
Place each head of iceberg lettuce in the bucket or basket. When you've harvested all the mature ones, or all the ones you can use within a week or two, bring them indoors.
Remove any discolored or broken outer leaves from the lettuce head. Wrap each head in plastic wrap and place it in your refrigerator.