I completed the process a couple of months ago so don't remember every detail, but roughly, this is what was to be done:
The following documents need to be sent:
The IRCC form: To give you an idea of what this looks like, the starting of the form contains your UCI number, Application number, name and then the following paragraph:
The processing fee. This was applicable to me because I did my biometrics at a different center than the New York one (where I eventually had to send my passport). I had to pay some $19 or $20 through a Western Union money order (which you can even buy from a 7 Eleven or some such shop) and then include that in your courier package
Contact info on a piece of paper, as mentioned
Consent Form. I believe it was this one for me: https://www.vfsglobal.ca/Canada/USA/pdf/USA.doc.pdf. But please confirm with the helpline if that's the one for you too. I think I had to send 2 copies of this - not too sure though. Also, I think this is to be filled up by hand in black ink.
A prepaid courier envelope that contains your address and name, etc. This prepaid envelope is used to send back your passport to you. This is something you can buy from UPS or USPS but NOT from FedEx because the Canadian Via center in New York does not accept FedEx return envelopes. (I bought it for around $7 from USPS)
This does not mean that you cannot send your documents in a FedEx envelope - it just means that the envelope that will be used by the visa center to send back your passport to you should not be a FedEx envelope.
Protip: Make sure the prepaid return envelope fits in the envelope you are sending your documents in. I did not make sure and had trouble cramming it in.
Important: If you have done your Biometrics in some other location other than New York (or wherever you will be sending your passport to get the visa stamp issued), then you will have to pay the processing fee.
Note: Their phone helpline is relatively helpful (+1 888-296-4511)
I would strongly suggest calling them and confirming the documents you need to send and ask them any questions if you have.
The helpline representative I spoke to also suggested that if I had any requests for the Visa center, I should send them a letter (in that same courier package along with my other documents).
When my passport reached the Canadian visa office, neither did they send me a message saying they received my passport nor did they confirm if they received it even when I called them up. I had to rely solely on FedEx's delivery person to make sure that the passport had reached correctly and then wait for the FedEx message saying my passport had been delivered. So it's very important that you have a good courier service that allows for tracking of your courier.
Take a picture of your passport/ get a photocopy done so that in case something goes wrong you have some backup for what its worth.