Petition for (Custody) (Visitation). These forms and instructions are available in the General Clerk's Office, Room 108, in the Nassau County.Complete General Form 17 to petition for custody or visitation. In this video we'll cover three terms to always include in the plan the steps to file for custody in New York and how to modify a custody agreement. A custodial parent may file a petition with their local child support agency. W ho is the "Plaintiff"? The "Plaintiff" is the person filing for custody. Contractors must also file a.