I did this over a year ago.
You 100% need the title of the car or else you will not get across the border. Ask me how I know. Stupid BMW dealer didn't send with car as they said they would.
Email your VIN and car make to
BuffaloVINNYOFFICE@dhs.gov at least 72hrs in advance. They respond with an email with more info. Don't need to send them anything else.
Whole process takes 30min across both borders. On US side you show them the title and sales receipt. They check VIN on the car and you drive across. On Canadian side you have to show your receipts and tell them you are importing the car. She writes up a one page document for you and you park in front of the building. Walk inside, show them papers, laugh when they realize you don't pay duty. Pay your HST and off you go. I am not kidding, it took me 30min in total.
Once all done you send the warranty recall letter to RIV, or send it before you import. Pay their fee and wait 2-3 weeks for your sticker. Go register car.
** I found RIV did not respond if my warranty recall letter was any good or not so I had to follow up with them a few times. It's funny since my letter was from BMW NA but on the top it said not to be used for exporting the car out of USA. Yet I had no issues. Try to get that letter as part of the car when buying. If you can't I can also dig up names of people who can get it for you for cheaper than the $500 BMW charges. I got mine for free with purchase.
If you have more questions let me know and I can dig up all my old emails. I have all my paper work still (except title, they take it when you register car).