I've been through the process successfully. Here is everything you want to know:
1) File PERM or Labor certification. Keep renewing your TN as you normally would. You can continue to travel and work using your TN.
2) After the PERM/LC gets approved, file your I-140. Do not file your I-485 concurrently. Renew your TN via mail if necessary (not at the POE/PFI) . You can continue to travel and work using your TN.
3) After your I-140 is approved, verify that you have at least 6 months left on your TN. If so, proceed to step 4. If not, apply for a new TN now.
4) Once your I-140 is approved and you have a TN that is good for at least 6 months apply for your I-485. Apply for your EAD, and your Advance Parole at the same time.
5) Do not leave the United States between step 4 and step 7. You can continue work using your TN, but you cannot travel using your TN. Never apply for a new TN ever again. If you leave between steps 4 and 7, your I-485 will be considered abanonded, and you won't be eligible for a new TN. Bascially if you leave, you can't come back, unless you get some other kind of visa, (H1, L1, etc) and you'll have to refile your I-485. Bad..bad..bad..
6) If your EAD does not arrive within 120 days after the application was filed, schedule an appointment at the local office using InfoPass and apply for an interim EAD. (this is why you needed to have at least 6 months left on your TN as mentioned in step 3.) I say 6 months, because the 120 days starts ticking after USCIS "recieves" your EAD application. If it takes them a while to get it into thier system that may be a delay. It may take a few weeks for your InfoPass appointment, even after you schdule it, depending on availability, plus, once you go get the Interim EAD, you may not be issued it on the spot. There may be an aditional delay of a week or so before you actually get it. So although you can "apply" for an I-EAD in 120 days, its better to leave a buffer of a few months on your TN in step 3.
7) Once your Advance Parole is approved, you can travel abroad. If you travel using your Advance Parole, upon re-entry to the US, you will be in I-485 pending status, not in TN status. Your TN status will be voided, and you'll have to provide your EAD to your employer to update the I-9.
8) Once your EAD is approved, you can find a second job if you so wish. If your I-485 has been pending for more than 6 months, then you can invoke AC-21, and leave your sponsoring company for another job that matches the description in your LC. Upon using your EAD, your TN is voided. All your employers would need a new I-9 with your EAD on it, and you would be in I-485 pending status, not in TN status. Your TN status will be voided.
9) If you don't travel in step 7, and you don't get a second job, or switch jobs in step 8, then you'll be in TN status until your TN expires at which point you'll be in I-485 pending status.
10) Keep renewing your EAD and AP every year at least 3-4 months before they expire so that you can take advantage of step 6 if necessary. Make sure they don't expire since although you'd still be in a legal status of I-485 pending, you would not be able to work or travel until the new EAD/AP arrives in the mail.
11) If your I-485 gets denied you would be out of status. At this point your EAD and AP are also invalid. You would need to apply for a motion to reopen as soon as possible. Once your MTR is accepted, you can work using your EAD again. (This situation is very very rare, so don’t worry about this one, and this is not at all specific to TN cases, since the same situation happen if your H1b had expired),
12) Eventually your I-485 will get approved, either directly from the service center, or you might get an interview like me. Your status will change from I-485 pending to permanent resident status.