Select two or three of your best lettuces, and mark them out for seed. It’s very important that you don’t collect seed from plants that bolt early as you want lettuces that will stand well. If your parent plants need a little help in getting their flowering stalks to emerge, try slitting the heads partially open with a knife as this often works well.
Once the lettuces have flowered, their seeds will ripen gradually and after about a fortnight you can begin to harvest the seed daily in order to get the maximum yield. This can be done by either shaking the heads into a bag or by waiting until a reasonable number of seeds are ready and then cut the plant away from its root. Put it head first into a bucket, then shake and rub it to remove the seeds. If you can leave the whole cut plant upside down in the bucket somewhere dry then any immature seeds that are left will continue to ripen over the next few days. Most of what you will collect in the bucket will be chaff, but you can sort the seed from it by shaking it gently into a kitchen sieve. Some seeds may fall through the holes but most will collecting the bottom leaving the chaff to rise to the top where it can be picked off. If the seed feels a little damp, leave it to dry on a ceramic plate before labelling and storing. Lettuce seed should keep for around 3 years providing it is kept cool and dry.