Check the Canada Post careers website. If there is a count going on in your area there will be a listing. Apply online. I sent my resume, as well as a cover letter explaining why I thought I would do well in the position.
There is no interview process as this is a temporary position. Instead, if you are accepted you will be contacted by email or by phone through HR and asked to supply scans of personal documents. This is used to complete a security check. If you pass this a few days later you will get another email with an offer! During which they will ask you to fill out, sign, and return a bunch of paperwork.
After all that all you have to do is show up to training and you are in.