Just finished this process. The information online is scarce, so here ya go, interwebz!

Needed documentation

Includes everything I've heard of, and everything I submitted, but you may not need all of this. Take it all if you can.

• [Required] Passport (obviously)
• [Required] Info about the company you're going to work for
• [Required] Info about what work you will be doing (Note - They also make you write this by hand, so be sure to remember it. It has to be activities authorized for the TN)
• [Required] Diploma & Transcript photocopies
• Original version of diploma & transcript (YOW actually didn't want this, but Toronto Pearson airport apparently does!)
• [Required] TN visa letter from your employer with both the start and end date (I had 3 years, they were happy with that) (Remember, this is a temporary visa, if you suggest anything at all that you intend to go for a greencard/etc you will be denied) (Toronto apparently wants this signed in pen, from the employer)
• [Required] Something that supports you are qualified for the job (for me I had a resume, and changed all the work experience to say "Software Engineer", which was my TN application category)
• Address you're staying at
• G-28 form if an attorney helped you draft these documents
• A document that explains why your degree is relevant to the job if it doesn't immediately match the job title (i.e. I had a Computer Science degree, so my attorney prepared a document explaining this was good for Software Engineering)

TN info at YOW

At YOW (Ottawa International Airport, AKA Macdonald–Cartier International Airport), the TN visa processing happens after security, at the U.S. Customs and Border Protection (CBP) pre-clearance. However, US security only opens at 4:30 in the morning, so do NOT book your flight before 7-8am!

How it went for me

• Arrived at 3:00
• Finished checkin at 4:30
• Finished security at 5:30
• Got to customs at 5:40, told officer I needed TN, officer told me I would miss my flight and I should have come earlier. I told him I was here at 3:00am, and he's like "Oh." Anyways, he took me to the "interview" room (or whatever it's called)
• Missed my flight at 6:00
• Waited until 6:30 to do the TN interview
• Paid 50$ (no cash accepted) for the visa
• Finished at 7:00
• The officers helped me book a new flight, which was incredibly generous and not at all expected or required

How it went for the person before me

Not super related to TN, but good info. This person was only intending to go on vacation, but was still pulled aside for advanced screening.

• Arrived at 2:00
• Finished checking at 4:00
• US Security only opens at 4:30, he was first in line
• He got to Immigration at 4:45. Since he had previously been declined a TN visa, they redirected him to the interview room (or whatever it's called) at 4:45
• For 2 hours, they could not verify he wasn't intending to illegally work in the US, so he was denied boarding

I overheard much of their conversation; his story is basically like this:

• He came to the airport 4 days ago and applied for a TN, and was rejected because he doesn't have a degree (Degrees are BS, but we live in a society...)
• He then came back today for a vacation, the border guards said sus and interviewed him. They asked if he had any documentation to prove he was not intending to work in the US during his stay (examples of proof would be documents proving he was laid off from the company after failing to get the TN, or documents from his new employer that said they had no relationship with the company that he previously tried to work for)
• Tl;dr he had no concrete proof, but he tried for 2 hours. Good on him

Some interesting takeaways from his discussion with the border guards:

• Allegedly, more people work illegally in the US than have criminal records, so the US officers are very nervous if someone has any hint of working without a visa
• It is ultimately the officer's discrecion to let the person in, and he was quite agitated, which the officers clearly did not appreciate (he called them pigs, among other stuff)

At one point, the denied guy, in a burst of frustration, loudly said "I want to go on this trip! You can't kill my dream!" and the officer was like "Well actually, that's kinda what we're doing".

The officer and I both looked at one another with the expression of "I'm trying not to burst out laughing". I'm a dick for that, but it was comedic.

TL;DR

TN visa takes a lot of time. Book your flight later in the day so you have time to wait, then get there early.

Feel free to ask any immigration related questions, I'll do my best to answer. The goal is to share knowledge publicly, so I won't respond to DMs!