Final answer:
Clerical instructions for extending military enlistment are typically found in official military personnel management manuals or historical service records, which outline the service terms and legal obligations of military personnel.
Step-by-step explanation:
Clerical instructions for the preparation of an agreement to extend enlistment in the United States Military can usually be found in official military personnel management manuals, historical records, or legal statutes that pertain to military service. Although a specific reference is not provided in the question, such information would typically be outlined in administrative documents which include enlistment terms, service requirements, and the legal obligations of serving personnel. Important sources for such information include historical service records such as the Record of Promotions, reductions, transfers, casualties, or legal texts that govern military service like the oath taken by service members that underscores their commitment to perform duties required by the law.