Take a whole head of lettuce with some roots still attached, and cut off the leaves to use for salad or sandwiches.
There should be 1-2 inches of green left above the roots
Find a nice spot of earth with plenty of sunlight, or put a pot of soil in the same area. Dig a hole deep enough to place the roots into comfortably.
Place the plant in the hole, and cover with the loose dirt from the hole. Leave some of the green sticking out of the hole.
Soak the ground around the roots, and I mean soak it, as the water helps the roots adjust and avoid shock. After the first couple of days just make sure the ground stays moist enough for the plant to grow well.
If the roots take well, after a couple of weeks your lettuce will have some lovely leaves sprouting off of its base. After a few weeks the plant will be ready for you to harvest a few leaves at a time to let the plant continue producing, or to harvest the whole head at once.