Thank you for your question.
The procedure you are looking for is located in FRCP 45: http://www.law.cornell.edu/rules/frcp/rule_45
You need to file a request to the clerk for the subpoena. You will need to know the exact location of the person you are trying to subpoena. Also, if you are trying to command the person to appear at the California court to give testimony, you may have to file a special motion with the court asking the judge to approve the subpoena. Otherwise, you will need to make the subpoena for a deposition where the person lives to take that person's testimony (the court does not like to compel people to come more than 100 miles from where they live if they are out of state).
Please let me know if you have further questions.
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