Welcome to FT, Dominic Campagna!

I'm afraid I don't understand your question. Do you mean (i) how do I enter my Aeroplan number in a flight booking, or (ii) how do I book a reward flight with my Aeroplan account?

If it's (i), go to the Manage My Bookings section of the Air Canada website, find your booking, and you'll see an option to add your Aeroplan number. If it's (ii) go to the Aeroplan website, log in using you Aeroplan number, and you'll see many options to redeem your miles.