Canada Immigration Passport Process starts when a decision has been made for your application. At this stage PR is almost confirm and CIC request passport to stamp VISA and Passport size photograph. Most of the instructions are well explained from CIC when they request for it.
Few notes I want to mention
1. Never try to send Old Photograph.
2. Make sure passport is not expiring any time soon should have at least 2 years validity.
3. Along with passport you will also receive COPR. This COPR is very important document for your immigration. After landing this will be regarded as your landing paper. Landing paper will be needed every govt. office you visit after you land to Canada.
Once you receive passport you start your shopping.