In summary, filing a Mechanics Lien in Illinois requires (1) careful investigation, (2) legal research, (3) detailed documentation, and (4) drafting documents carefully and accurately, in accord with the facts and the law of each case.
Illinois provides a powerful set of remedies for contractors who have a valid mechanics lien, but the other side is that if you do it incorrectly, the remedies for the owners are equally harsh.
Filing a Mechanics Lien is not a simple process. Despite what some people will tell you, it is not simply a matter of filling out a form. Filling out the form incorrectly, or not having your facts organized correctly can lead to consequences far beyond a minor inconvenience. Getting the information wrong, signing, and having your signature notarized may very well get you convicted of perjury; that's not just a crime, that's a felony. Prosecution may be rare, but why risk the trouble? Filing an invalid or fraudulent lien can also get you sued for slander of title, if you wrongfully prevent the owner from selling or refinancing the building.
Even if you don't get prosecuted, presumably you're filing the lien because you want to get paid. The requirements for correctly filing the lien must be followed precisely, or else you make your lien invalid. Getting it wrong simply isn't worth it - you have to get it right to get paid.
This site provides information that can help you understand the Mechanics Lien process in Illinois. However, the process is complex and often requires the assistance of an attorney. Consider hiring an attorney - your "legal subcontractor" - to complete this specialized part of the construction process.