Hello 1nfrequent. The procedure is very straightforward. Queues can be long, but that’s another matter. I assume that you’ll be checking baggage. Whether you check in on-line, at a kiosk in the terminal, or at a check-in desk, once your bags are properly labelled you’ll make your way to the US Pre-Clearance hall (the entrance to which is easy to identify as there is usually a large US flag hanging from the ceiling). As noted above, it’s very helpful if you’re able to complete the document(s) before arriving at the airport (I always keep a handful in my desk). If you cannot do that, that’s fine. Just be sure to have a pen handy. As mentioned above, you’ll take all baggage (carry-on and checked) to the Pre-Clearance hall. They seem to have staffing issues much of the time, so best to arrive with plenty of time to spare. Remember: you must queue up twice. Once, for the US Pre-Clearance, and then again for security. It always amazes me how many Canadians (who know the procedure) think that arriving sixty minutes prior to departure is sufficient. It is not. But back to the staffing shortages. Here in Toronto during the morning rush (approximately 5am-8am), the Pre-Clearance area has become so congested that they’ve actually started sending passengers to a holding lounge where you’re to wait until your flight is called. Only then can you proceed to the Pre-Clearance area. If your flight is in the early morning then I suggest you allow at least two hours for the whole process (best to be safe than sorry). After you’ve had your turn at the Customs Officer’s desk, you’ll be able to place your checked bags on a conveyor belt and then proceed to security with only your carry-on. It’s well worth the extra step in Canada, as it’s wonderful to be able to deplane at your US destination and carry on as you please.
You might be travelling at a non-peak time of day, in which case you might not have to wait long. However, I recommend taking a paperback novel (or similar) for what could be a lengthy wait in the Pre-Clearance area. Cell phone use is generally not permitted there, although the rule is not always enforced. As long as you arrive early, with something to read and a good dose of patience, you should find the experience to be painless.