You normally use the standard subpoena form to subpoena a police officer, firefighter, or other peace officer to testify about anything observed or investigated in the course of duty. Fill the form out, take it to the court clerk for a seal and date stamp, then take it to the relevant police or fire department to comply with any of their additional procedures. (For example, the police department may require two copies of the subpoena instead of one.) Give the witnesses plenty of advance notice. You will have to pay the officer's salary for the time taken off work to appear. You must pay this in advance in the form of a deposit (usually about $150), payable to the agency employing the officer. The subpoena may be served on the officer personally or on the officer's superior.
Depending on the amount of the officer's time that is used, you may eventually get a refund of some of your deposit. Don't hold your breath, however. Many law enforcement offices are seemingly unaware that this is a deposit and not a flat fee, and have no procedure in place for issuing refunds.