The first step in appealing your property tax value is to appeal to the county Board of Equalization & Review ["BER"]. Each county has its own BER. So, one obviously appeals to the BER in the county where the property is located.
In order for the appeal to be timely, it must be filed by the "date of adjournment" of the BER. The date of adjournment is a somewhat artificial date, as the BER will continue to meet and hear appeals after its "adjournment date." Nonetheless, to be timely, an appeal has to be filed by the adjournment date.