With Pre-Clearance:

1. Check-in with your airline, either via OLCI or at a kiosk at the airport or in-person. Receive your boarding pass.
2. Fill out US Customs Declaration Form.
3. Proceed with your boarding pass, and all your bags for the journey, to the Pre-Clearance Area. Depending on the airport (for example, YVR), drop your bags off at the baggage carousel before proceeding into pre-clearance area.
4. Proceed through USA CBP formalities - immigration and customs.
5. Proceed through Pre-board screening.
6. Purchase duty-free, depending on airport. YVR/YYZ has duty-free shops AFTER you clear CBP formalities and security.
6. Board your flight to the USA, with duty free goods if any in carry-on.
7. Land in domestic terminal area in USA airport. If transiting within same terminal, no further security checks are usually required.