You can plant its seeds or buy in plant form.  Choose a species of lettuce you like most.

Sprinkle the seeds over the soil as you would if you were adding fine salt to your meal. Cover the seeds with a thin layer of potting soil (3 – 5 mm).

Moisten the seeds every morning.  The earth must be constantly moist if you wish your seeds to grow.  In a few days time you will see your first small plants.

Pick off the leaves you need either gradually or all in one go.  As soon as the leaves reach the size you desire (already between 4 – 5 cm) you can begin to pick them off, starting from those further out.  Leave the inside leaves on so they can continue to develop.

If you wish for a fully mature lettuce head you must trim them leaving 7 -8cm at the base.  The removed leaves can be consumed.