In addition to passport and Annexe A (is a part of the PPR e-mail), you need to send
-two photos of yourself (the large kind
https://www.canada.ca/content/dam/i...ormation/applications/guides/pdf/5445eb-e.pdf).
-Canada Post Xpresspost envelope for return mail.
That's about it, those are the two things you can prepare beforehand. I sent my passport+photos+Annexe A + Xpresspost envelope in a normal envelope to Ottawa and it was fine. You can use Xpresspost as well, but since I live in Montreal, the normal envelope reached Ottawa in 1 day as well.
Edit. Do NOT fill in the address field in the Xpresspost return envelope. Leave it pristine, blank, untouched. They are very specific about this because they will print their own stickers