This is what CIC says in its website:
"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)."
Link: http://www.cic.gc.ca/english/resources/tools/perm/express/intake-complete.asp