Check the proof of work experience section under
http://www.cic.gc.ca/english/resources/tools/perm/express/intake-complete.asp
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if the applicant is self-employed, articles of incorporation or other evidence of business ownership, evidence of self-employment income and documentation from third-party individuals indicating the service provided along with payment details (self-declared main duties or affidavits are not acceptable proof of self-employed work experience).
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I also submitted a standard reference letter signed by a manager at my company, salary tax documents (personal and for the company), office rental documents, a few invoices with client information and a detailed letter of explanation