It's been noted in other posts, but, the one step that is missing is to notify the border crossing you intend to cross at with all your information, either by email, fax, or, dropping it off in person.
For me this was Blaine (Pacific border crossing), don't expect different crossing to have the same policies and procedures
I needed to fill out a worksheet, only found by accident here:
http://simplifiedtradesolutions.com/wp-content/uploads/2014/10/BlaineCBP-ExportWorksheet.doc (Blaine border crossing vehicle export worksheet)
I also sent as attachments the document with my ITN, a copy of my title, and the worksheet. I also put my vehicle description, VIN, and title number in the body of the email. This was accepted by them and I got a reply email stating when I would be clear to cross with my vehicle.
They also have a little bit in the reply email about how your vehicle must remain outside of Canada for the 72 hours and there are penalties and fines and they can seize your vehicle. In the end they never asked me, but, I jumped through hoops to make sure my vehicle was in the US for the period.
The last bit of info is, what to do at the border!
Act like you're driving to the Canadian border crossing (you will be right after). Park in the hi-lighted red area, there is a shack with big yellow letters that says KIOSK, that's the right place. Leave your vehicle unlocked and make sure your dashboard VIN is visible from outside. If you make a border guard walk outside twice to check you vehicle you'll catch shit. Don't park in visitor parking, they'll make you move it before they do anything.
It's weird walking across all the lanes of traffic, but nobody will stop you. The export window is inside on the left 3/4s of the way down the wall, it's just one little window. It's the little plus in my diagram.
Good luck!
If there is information you needed that was missing, signup and post it! I know you're done and want to forget about ever dealing with this shit, but, our experiences are the best documentation for this process!
