It's pretty easy, the basic details for me in the US was a 7 day free trial, and then $9.95 a month with 5000 mins a month calling back and forth between Canada and the US.
I chose the area I wanted the phone number to be from, they assigned me a number. Then I had to download their app onto my iPhone. It works almost identical to the regular iPhone. You can also put the app on iPads etc. My 5yr old son has it on his iPad and has been making use of it. Why, I don't know when he has the ability to Facetime me. I guess it's the novelty of having a "phone" of his own ;-)
I think you also have the ability to forward the calls to your phone directly for incoming calls. Anyways, when I called CIC the number was recognized as Canadian and I was able to get through to an agent.
This is who I used, hope it helps:
https://www.line2.com/
If anyone knows of a similar service that is better or more affordable, I'd love to know.