If you are just going to flagpole, take your passport (it should have your visa sticker and your COPR), any other docs requested in the letter they sent back with your passport, and your list of goods to import/follow, if any. Tell the US border guard your purpose is to officially land in Canada. They will tell you how to turn around to get to the CAN customs and immigration building. Inside you will answer some questions, confirm your identity, declare the address for mailing your PR card, get some papers (your COPR which you MUST keep plus some how to docs for getting SIN, OHIP, etc, etc). This takes about 10 minutes. Then you move on to customs to declare your belongings. Then you are done. Hopefully, someone will smile at you and say "Welcome to Canada".