You are required to carry the Letter of Introduction along with all the specified documents to Canada. At the airport, you would meet with an Immigration Officer.
Present the Passport and the Letter of Introduction to the border control agency officer who would then validate the same and if found adequate, issue you your Study Permit.
For staying in Canada, students need to have a valid study permit. If in case, your study permit expires before completion of your studies (program) then you need to apply for renewal of study permit. Ideally, you should apply at least 30 days before your current permit expires. Candidates can submit their applications through mail or online.