You can use your National Entitlement Card to purchase your discounted travel ticket either before you travel, or once you board.
Most buses are now equipped with electronic ticket machines which read your card, however if your bus does not have one of these machines just show your card to the driver.
In order to gain the discounted travel, you have to use your most recently issued National Entitlement Card. Previously issued cards will not work and you will not be allowed to travel for free.
If you find that you have more than one card, you can check which is the most recent by looking at the last two digits on the long card number. Card numbers will end in 01, 02, 03 etc.