It is very easy!

You only need to prepare all the documents they ask for, and mail to Canadian Embassy in Beijing, then, they will give visa to your parents and mail them back to your parents( after about 1 and a half month). Your parents can also go to Beijing and submit the documents by themselves ( they don't need to have interview, just hand in the documents), then your parents can get the visa at the same day.
I send you an email, if you have questions, you can email me.