Just let the whole plant dry, then harvest the seeds. Be aware though,
that your seeds will not give you the same type of lettuce you grew
this year if it was a hybrid. If this is the seed you got at the Dollar
Store that you mentioned in another thread, this will be a crap shoot.

There are several leaf lettuces available that will breed true. Doing a
Google search for 'heirloom leaf lettuce' will get you several types
and sources. Black Seeded Simpson is one good heirloom lettuce for our
climate.

Since you're more interested right now in getting seed rather than
leaves to eat, you could plant them now and maybe get your own seed in
time to try for a fall crop.

BTW, I hope you're not throwing out your spent lettuce and spinach.
Just turn them in, or let it decompose on the surface. Compost is your
friend, and your earthworms will thank you!

David