response to subpoena duces tecum

Since you are a non-party to the litigation you have no procedural obligation to respond with any specific form. Your only obligation is that you comply with the subpoena (assuming you have no objections to the requests contained in the subpoena.) You can merely write a cover letter explaining what you are producing. Make sure you get a receipt to prove you complied, such as a return receipt from the Post Office. By the way, you can also charge the requesting party for the reasonable cost of the copies and postage.