In the last 2 years I have imported a good number of sleds and trailers. Enough that the Canadian Border told me that I cannot show up again without a vehicle business import licence! My bad

!!
You do not "have to" do anything with the U.S. side if you do not want to. It is recommended, but not manditory. Call the Canadian Border Services and they will tell you the same thing. The reason you "should" contact the U.S. side is for them to have time to run the VIN to verify there are no liens against the vehicle and that it has not been involved in a theft or crime.
The Canadian side really doesn't care whether you do this check or not. I did it for the first few sleds I brought accross, but certainly not all of them. I personally asked the Canadian side and that is what they told me. It is for your own good to do it though, unless you know the seller well and can take his word.
You do need the recall letter if you wish to licence the sled for trail use in Canada. Do not bother with Canadian dealers. It is a huge money grab in my opinion. I would go to any Doo dealer near where you buy the sled and ask politely for the recall letter. I would offer $20 for their time and trouble. Most would not accept it, but appreciated the kind gesture.
DON'T TRY TO ALTER OR FUDGE THE PURCHASE PRICE!!! Been there, been caught, don't bother!!. My bad again

!! If they get a bee in their bonnet and want to take a closer look at your stuff, they will dig until they get what they are after. You can be asked for your bank statements, the ad from where you found the vehicle, the contact information of the seller and the list goes on...... It is not worth the few bucks you save in tax. I blame no one but myself for my encounter with the border folks, but if I can save you the hastle, it can have some good come out of it.
On the way down, I would bring a copy of the ad you found the sled in, if you want, the bank reciept for your withdrawl or money conversion, and the contact info for the seller. If you want to do the 72 hours notice for the U.S. side, you need to get the seller to either fax you a copy of the title or get him/her to fax a copy directly to the U.S. Border Patrol 3 days before you plan on taking the sled accross to Canada. You can call them and they will give you the fax number and maybe a "send it to" person or dept. They require the title to be sent 3 days in advance. All they do is run the VIN and if it is all good, they will emboss your paperwork to validate the check has been done. If the buyer feels uneasy about this step, remind them that it does not mean they HAVE to sell you the sled. You can fax that stuff in and not show up with the sled and the U.S. side will not care.
Again, if you are going to do the U.S. stuff, keep in mind that the U.S. side for processing this kind of paperwork is not always open 24/7. Last year, the Detroit location was, but the Port Huron/Sarnia was regular business hours only and NO WEEKENDS!!. I would call the border you plan on crossing at and verify their hours. They get very unhappy if you show up for a check after hours. Yup, I know that too
Outside of that, it is a simple process as stated by the other DooTalkers. You tell the fella at the Canadian booth what you are doing, he wil tell you to go to the main Border office and you will be greeted by one or more Border Police. Tell them what you are doing and they will likely look at your machine and your paperwork briefly and then send you inside. There you go to the counter and present your paperwork. They will fill out the RIV form and get you to sign it. You will be charged the just over $200 RIV fee and one of the sales taxes. Can't remember which one. You will pay the other sales tax when you finally register the sled at the MTO. Once you pay your fees inside the Main Border office, you go outside and present your paperwork to a border police officer and he/she will take a closer look at your sled and maybe your vehicle too. Make sure you do not have stuff in your vehicle that should not be there or that you did not claim, it can get ugly.
When they are all happy, you go home. At the border desk, you can register to be notified by email that you RIV paperwork has been completed and is clear. Make sure you do that. IMPORTANT: RIV Canada will NOT call you if there is a problem with your paperwork!!! It will sit dormant until you finally get so frustrated that you call them. Then they will tell you the issue/s that you need to correct. I can't remember how long it normally takes for the paperwork to clear, but once you find out, call them the day it should be done, or before, and ask if there has been any issues. It will save you a pile of time.
when it is done, you will get an email to print out. Call your local Canadian Tire and book an appointment for an RIV inspection. Bring ALL of your paperwork with your sled. As mentioned, your VIN must match and your lights must work in order to pass the inspection. They will stamp your paperwork and you are good to go to the MTO and get the machine in your name. You will pay the other tax at this point, as well as your licence fees.
Some time later(weeks) you will get a letter in the mail that will contain a new i.d. plate sticker that goes on your tunnel. It is basically the new i.d. plate for that sled in this country from that day forward.
All the above applies to trailers too. Anything you licence for the road or trail needs to go through this process. Once the vehicle is 15 years old, there is no RIV fee or process for it. The border will stamp you paperwork for you and off you go.
Feel free to contact me via a pm if you have any questions. I will be happy to chat in person too, let me know and I will shoot you my number.