1. Add baggage on the website of the airline
If available, the option to add baggage can be found there and usually (extra) baggage can be paid for directly.
You have to sign in on the website of the airline in order to add baggage. Please go to 'Manage my booking' and fill out the required details (your booking reference can be found in your ticket e-mail).
2. Add baggage at the airport
At the airport go to the desk of the airline and ask the ground staff to add baggage and pay for the baggage directly.