Part of the fun of having a garden is getting to eat what you grow. You have waited all season for your lettuce to be ready, so you certainly do not want to mess it up by making a mistake picking it. You know that some things in the garden, carrots for example, it is okay to pull right out of the ground. Other things, like tomatoes, need to be trimmed off of a plant. Lettuce is a bit tricky, though. There are some types that need to be pulled from the ground and other types you can just trim off as you need.
Wait until either your romaine lettuce forms an inner heart or your butterhead lettuce forms a sizable head. Gently push the leaves of the lettuce aside and grasp the lettuce around the base of the leaves. Wiggle the lettuce a little bit to loosen the ground around it. If the earth is packed in very tight, you might want to gently turn it a little bit with a small shovel or rake. Holding on to the base of your lettuce, lift it out of the ground without disturbing any nearby plants. Shake the lettuce to loosen any dirt that might be clinging to the roots. Break off the root ball and dispose of it or compost it.
If you are growing loose leaf or baby lettuces, you will harvest your leaves more like you harvest tomatoes, one by one. Allow each leaf to grow to about five inches long. When the outside leaves reach an appropriate length, you can cut them off with a sharp knife. The leaf will grow back and the crowded inside leaves will get some sun and air. You can also harvest an entire plant's worth of lettuce at once by grasping all the leaves together and cutting them off just above the base of the plant. This should result in your lettuce growing back.