You likely need to contact your US border crossing 72 hours in advance. You can find instructions about exactly how to do this here
https://help.cbp.gov/app/answers/detail/a_id/307/~/requirements-for-exporting-a-vehicle-%28including%3A-snowmobile,-atv-and-motorcycle%29 Drive the car back to the US, go to the export office (don't show up early, wait at minimum the full 72 hours). They will require your ID a passport will work fine and the original title to the car. Do not make photocopies of your original title even though the extra sheet that you downloaded and filled out says to bring in 3 copies of your title. Don't worry about that part of it. They will process it in their computer, then they will give the title a stamp and turn you around and set you right back to Canada.
If you have already processed your B4 with your car listed and they have given you the RIV Form 1 then you will just go through the border normally. If you have not already processed your car and have possession of RIV Form 1 then you will need to stop in at your Canadian immigration once you cross back into Canada. They will get your information, look at the title of the car, so on and so forth and give you Form 1.
You will then fax/email that form along with a copy of your recall clearance to RIV. If you can obtain recall clearance online, they will USUALLY accept this, if you must get a manual letter, it can not be older than 30 days when you send it to RIV. Once RIV had gotten your Form 1 along with your fee and your clearance letter they will send you, via email (quickest route) or snail mail Form 2. If you use email double and triple check the email address you provided them as they don't verify it and if you put it in wrong you won't get your Form 2.
You can then make an appointment with Canadian Tire (or whomever is designated to do import inspections where you live). Take both form 1 and form 2 along with the car and have it inspected. It is usually a very short and sweet inspection not involving a whole lot other than looking to ensure all lights work, you have daytime running lights and taking some information from the manufacturers sticker located on inner door sill.
You can also opt to have them complete your emissions testing at the same time. I did not as my local garage was better priced for this.
Once you have your emissions testing done and your RIV inspection done, you can then head to your local service (enter province you are from here). I live in Ontario, so I went to Service Ontario. I gave them all the paperwork and they gave me a registration and title for my car which I then had to take to my insurance agent. The insurance agent then insured my car, and I took this insurance card (proof) back to my local Service Ontario. Once I showed them proof of my insurance they then finished processing my car and gave me license plates.
Voila, car imported into Canada. Roughly 10 days later I got a letter from RIV in the mail. It contained a door sill sticker and instructions on where to place it in the car. I put it just below the original manufacturers sticker on my inside door sill.
I hope this helps you to understand the required steps to get your car into Canada. It doesn't matter if the car has been in Canada already, btw. Mine was stored in my garage here at home in Canada all winter. I just drove it back into the US to have it exported. I told the guard that I had it in storage all winter and am now importing it since I got my PR. There was not a problem with this.
