Start with the property record itself. An assessment built on the wrong square footage, an extra bedroom that does not exist or a misclassified secondary suite will overstate value on its own, before comparable sales even enter the picture, and that kind of error is corrected rather than argued.
If the record is accurate, line up three or four similar homes that sold recently and see where your assessed value falls relative to them, since residential property is taxed on 80 percent of assessed value and even a modest gap there moves the bill.
A bill that tracks the market is not appealable no matter how it feels; a bill built on a wrong number is. Get your hand-reviewed valuation from Joel Dyck.