Get the timing right before anything else. Appeals must be filed within the window printed on the assessment notice, and that deadline is strict, so read the notice the day it arrives rather than the week the taxes are due. Check the property details on the notice first, because an incorrect square footage, basement development or lot size is the most common and most fixable ground for a successful appeal.
An appeal is an argument about value, not about the size of your tax bill or how the rate compares to a neighbour's. What persuades is evidence: sold comparables around the relevant base date, documentation of condition issues, and a clear explanation of why the assessed figure exceeds what the property would sell for. Note that a market value opinion from an agent is not a formal appraisal, and for a larger or more complex property the Board may find an Appraisal Institute of Canada report more persuasive.
The report Joel prepares uses comparable sales and a walkthrough of the property. That matters in both directions, because public records can as easily overstate a home as understate it, and an assessment built on data that has never seen the inside of the house is exactly the kind of number worth testing.