For California small claims, you don't have to “serve” a demand letter. You just have to send the demand letter. You can send it by mail, email, or hand delivery, or you can text the demand letter.
Here's a short list of what you'll want to include: Facts of the case. An outline of what happened. Statement of the issue. A brief description of the problem. Demand. The dollar amount or action necessary to resolve the case. Response deadline. The date by which the recipient must respond. Noncompliance consequences.