After booking a flight with Air Canada, you can choose your seat by following these steps:

1. Visit the Air Canada website and click on the "My Bookings" tab located on the top of the home page.
2. Enter your booking reference number and last name in the fields provided.
3. Click on the "Find" button to retrieve your booking information.
4. Select the flight for which you want to choose your seat.
5. A seat map for your flight will be displayed. Choose your preferred seat by clicking on it. You can view details about the seat, such as legroom, by hovering over it with your mouse.
6. Once you have chosen your seat, click on the "Confirm Selection" button.
7. If there is an additional fee for the seat you have chosen, you will be prompted to pay it before you can confirm your Air Canada Seat Selection.

If you encounter any issues while selecting your seat, you can contact Air Canada's customer service for assistance.