Seed heads will form on long stalks on lettuces. Allow them to mature. After they have flowered, the flower buds will close. Harvest the these seed heads once a large number of these flower buds have a cottony appearance (think dandelion heads). Place the seed heads in a paper or cloth bag and allow to fully dry. To harvest the seed rub the flower buds and the seeds will fall out. Use sieves to separate the stalks and flower pieces from the seed and/or winnow by placing them in bowl and blowing away the unwanted pieces. Store in cool dry conditions. Lettuce seeds remain viable for at least 5-6 years.