Easy peasy. When you arrive in YOW, walk towards the exit, but take the exit on the right and not the one on the left (ie. not the exit that goes down the escalator). Take the one on the right to stay on the same level, walk straight a bit and then take the staircase in the middle of two escalators. When you're upstairs (Timmy's should be in front of you), turn left and left again and walk to the end of the departure hall. U.S. Departures are there.