Growing lettuce indoors is a fairly easy task, as many varieties of lettuce are hardy enough to survive in less-than-ideal conditions. Lettuce does require a fairly cool environment to flourish; it does best in places that reach 60 to 70 degrees Fahrenheit during the day and remain above freezing at night. The vegetable also needs ample light.
Wash and dry the plastic clamshell containers thoroughly, then use a sharp knife to cut about ten small drainage slits in the bottom. Some of the best examples of these containers are the clear, lidded boxes that contain gourmet salad greens in stores. They are large enough to allow the lettuce to expand.
Pour about 2 inches of the lightweight potting soil mix into the container. Using a light, airy soil is important because it drains more easily than standard potting soil. It also allows the lettuce to breathe, which is particularly important when growing plants in plastic containers.
Scatter a pinch of lettuce seeds on top of the soil, then cover them with a dusting of dirt. Spritz the top of the soil with water from a spray bottle until the dirt is very moist. Pop the plastic lid on the container.
Place the container in a warm, well-lit spot. If you live in a very cold climate, do not place the lettuce near windows; it could freeze. If you do not have a safe, bright spot for the plants, you may want to invest in a florescent grow light. The lettuce should sit under the grow light for about 12 hours daily.
Remove the top from the container after the seeds have sprouted, a process that usually takes about 5 days. Nest the bottom of the container in the lid, so that the top becomes a watering try. Fill the lid with water every day to water the lettuce; watering the plant from above can rot the leaves as the lettuce grows lusher.
Harvest the lettuce 3 to 4 weeks after planting. Use a pair of scissors to clip the leaves away from the plant, allowing the roots and crown to remain intact. The lettuce should grow back about 2 weeks later, when it can be harvested again.