Canadian airports with US immigration and customs pre-clearance facilities will have the US customs forms at stands next to the checkin area of the departures-to-the-US (may be called "transborder", or marked with US flag icons on signs) terminal.

The order is this:

1. Fill in US forms.
2. Check in (if necessary) and tag baggage to check (keep it for now for US customs).
3. US immigration pre-clearance.
4. US customs pre-clearance.
5. Put checked baggage on baggage belt.
6. Go through security into the airside part of the departures-to-the-US terminal.