There are a couple ways to do this.

1. You can just take the outer older leaves froma given plant. And it helps to have enough planta to rotate or spread the teimming across some or all of them. They will recover faster.
2. You can trim the entire head about 1″ off the ground. This will knock back the plant sharply, and they may not recover, but there is avery good chance they will regrow and you can do thia again in a few weeks. See the guide below.

[1]

As you can see in the image, there are many plants one can do this with and not just lettuces but mustards, kales, chois, collards, etc.

Happy gardening.

Footnotes