You go through the main Treasury Board portal and specify cbsa at the appropriate stage.
https://www.canada.ca/en/treasury-b...y/access-information/request-information.html
I find most of it reasonably self-explanatory, the two things to be clear on is you are looking for privacy act (personal records, not corporate information), and you are not an employee looking for employee/employer records. I think once you understand that part should be reasonably clear. In explanatory you can clarify you're looking for all files relevant to citizenship application. It may take a bit less time if you specify time period that corresponds.
