You parents must to be living in Canada for over 183 days per tax year.
1,Letter to explain why you need apply HST return
2,T1261
3, T1 General(your parents)
4, Passport photocopy(page of the following photo, VISA, custom stamp)
Registered letter to:
International Tax Services Office
Returns Processing Division
2204 Walkley Road
Ottawa ON K1A 1A8