Easy process, at least it is if you already have US Global Entry. I was in and out in 15 minutes. They don't tell you where to find the office in Pearson though. It's on the Departure Level, in Terminal 1, between numbered check-in stations 8 and 9 - closer to 8. Look for "Nexus Office" as opposed to the way through immigration for those who are already Nexus approved.