"Instructions for Service of Subpoena" tells you how you must legally deliver (serve) a subpoena to someone so the court will consider it valid—for example, whether you must hand it to them personally, or use substituted or conspicuous service; who is allowed to serve it; and what steps you need to take so the subpoena compels someone to testify or produce records in your case.

Following the correct steps for service means you can later prove to the court that the subpoena was properly delivered, which is required before the witness or records can be enforced.