I assume you are the former tenant?
Serve the letter on the landlord.
It is not essential, but highly recommended that you either send the letter via certified mail, or ask for a receipt if you deliver it in person.
Here is a sample:
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, consult with your own attorney.