First you must collect evidence of over-assessment. Then submit a complaint to the Board of Assessment Review by the Grievance Day deadline along with your evidence. The Board of Assessment Review considers your case and makes a determination, usually within 3-4 months. If the Board agrees with you, your assessment is lowered and you are done. If they don't agree, maybe you get a small reduction or possibly none at all.
You can appeal the Board of Assessment Review's decision by filing a petition with the Small Claims Assessment Review. File copies of your paperwork with the assessor, clerk, school district and village, if applicable. Schedule a negotiation with the assessor prior to a formal court hearing. If you and the assessor can't agree on a reasonable value then you must schedule a hearing with a Small Claims Assessment Review hearing officer. You would then have to argue your case in front of the judge, providing evidence and further support that you are correct. The judge renders a final verdict within 30 days of the hearing.