I used this yesterday at T3 (well, Lyft, as they have coupons on airport rides right now). Directions to the Ride Share area was clearly signed. It's outside Door D. There's three zones (1, 2 and 3) and you stand by the one you've provided to the app.

It was very quiet there as not many people were using it, which was a welcome break as YYZ is normally heaving.