These forms and instructions are available in the General Clerk's Office, Room 108, in the Nassau County. Petition for (Custody) (Visitation).Do I need to file a petition or should he? Basic Steps to filing for custody in Cook County, IL. Preliminary Matters. Step 1: File Initial Petition. The forms below are some of the most common forms used in the Domestic Relations Division, but there are many more. Some forms in this category are now available as Fillable Smart Forms. Below you will find all forms in PDF format. Fill out parentage forms. First, fill out and sign the forms listed below.