To register your frequent flyer number, you should first sign up for the airline's loyalty program through their official website to receive a unique ID. To ensure you earn points for future travel, log into your account and add the number to your profile settings, which will automatically link it to any new bookings made while logged in. For existing reservations, you can use the "Manage Booking" tool on the airline's site or app to manually enter the number before you fly. If you have already flown and forgot to add it, most airlines allow you to retroactively claim miles within 6 to 12 months of the flight; you will simply need your ticket number and boarding pass details to submit a "missing miles" claim through the "Loyalty" or "Frequent Flyer" section of their website. Always ensure the name on your ticket matches the name on your loyalty account exactly to prevent registration errors.