Originally Posted by Jebby_ca

LH does not participate in the international > USA connection process.

You'll have to:
1. Clear Canadian Immigration.
2. Collect any checked baggage.
3. Clear Canadian Customs.
4. Exit the Customs Hall, and head for the elevators to your left. Take them up to the Departures Level (Level 3).
5. Turn right as you exit the elevators, and walk all the way down to the end of the US Departures Terminal. Alaska is at the end of the concourse.
6. Check in with Alaska, and have your bags re-tagged if necessary.
7. Drop off your checked luggage before you enter the US Pre-Clearance area.
8. Clear US Pre-Clearance security.
9. Clear US Immigration and Customs.
10. Head down to the Alaska Q400 gates, which is on your right after the Duty Free shop. It's a good walk.

Depending on your arrival time into YVR - Canadian Immigration/Customs can take quite a bit of time, depending on what other international flights land at that time. If you don't have Nexus, I'd keep about 2-3 hours to clear that process above.

Hope that helps!