I've used the Report Debtors service offered through MR. Landlord. It is fairly straight forward. You need the basic information: name,SS#; last known address, etc.; the amount of the debt and the the reason for the debt. You also need the date the debt began. There is a section where you can add a comment if you need to elaborate on anything.
As for your word vs. theirs - if the debtor disputes your claim then the CRA will investigate and at that point you will need to substantiate the debt and prove that the information is accurate. A lease and some correspondence or judgement showing the debt exists should suffice.
It is always good business to keep complete tenant records regarding payments, etc. for several years especially if you plan to file against them.
One last thing - you need to notify the tenant (past and present) that you are attempting to collect a debt. You need to send this notice (exact wording provided by the reporting service) 30 days before you report a bad debt to the service. The notice doesn't have to be received just prove you sent it.
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