Deducting employment expenses depends on a form your employer signs. Form T2200, Declaration of Conditions of Employment, must be completed by employers in order for their employees to deduct employment expenses from their income.
What this form is
A declaration about the job, not a record of what you spent. It certifies the conditions of your employment that sit behind the claim. Your receipts show the money; the T2200 speaks to the conditions under which it was spent.
Who fills it out
Your employer. The form is theirs to complete and sign; you cannot fill it out for them. The Canada Revenue Agency (CRA) publishes it as a fillable PDF and a standard or large print PDF, and alternate formats such as digital audio, electronic text, braille, and large print can be ordered.
Employers complete a T2200 for the year in question, and the CRA keeps the form for past years on the same page, so a form signed for one year is not a form for another.
What it supports
Employment expenses, on line 22900 of the return. The declaration is the condition behind the claim; the amounts come from your own receipts and are totalled when you file.
What to keep
Keep the signed form with the year's receipts. It is kept, not filed — you do not send it in with your return, but the CRA can ask for it, and a claim without it is a claim you may not be able to support.
Get a new one each year you claim. Because the form describes the conditions of a particular year's employment, last year's copy does not carry forward.
Dossier keeps the signed declaration alongside the receipts it covers, so the year's employment expenses and their support sit together when you file.
Official CRA sources
Source: Canada Revenue Agency (canada.ca). Not tax advice.