Lettuce seeds are tiny things, some of them no bigger than the head of a straight pin. Because of this, it's often easier to scatter them in a tray for sprouting than to pick them out one by one for planting in segmented trays--a difficult task for even the smallest-fingered among us to manage. Once the seeds have germinated, and the seedlings are about 1-inch tall, they're ready for transplanting into your garden.
Choose a free-draining seed tray that is at least 1 1/2- to 2-inches deep. Fill the tray to within 1/4 to 1/2 inch of the rim with a soil-free seed-starting mix. Ground-up sphagnum moss is good for this.
Pinch some lettuce seeds gently between your thumb and forefinger and scatter them thinly across the surface of your planting medium. Add another thin layer of planting medium or compost on top of them and very gently firm it down with the palm of your hand. Water the starting mix so that it's thoroughly and consistently damp.
Cover the tray with a plastic dome (if it came with one) or with a sheet of plastic wrap. Use tape if necessary to help attach the plastic to the tray.
Check your lettuce seed tray every day and mist with water from a spray bottle as necessary to keep the soil damp. If you're in a very dry environment, you may need to add more water. Take care to dribble it around the seedlings as they emerge, since a "mass watering" would crush many of the delicate plants.
Remove the plastic dome or sheet that you applied once the seedlings are tall enough to touch it. If you leave it in place their growth will be stunted.