Head to your device settings and click into the 'Wallet & Apple Pay' section. Click the 'Add Card' button and follow the steps as outlined.
Step by step...
Select the 'Settings' button on your home screen.
Scroll down and select the 'Wallet & Apple Pay' section.
Select the 'Add card' button in the PAYMENT CARDS section.
Select 'Debit or Credit Card' option.
Select 'Continue' button.
To add your card, you can either scan a picture with your camera, or select the 'Enter Card Details Manually' button at the bottom of the screen.
7. Once the card details have been added, check/verify to ensure the details are correct. If they are, select 'Next' in the top right corner.
8. Enter the cards Expiry Date and Security Code (3 digit number on the back of the card). Once complete, select 'Next' in the top right corner.
9. Once reviewed, select the 'Agree' button on the bottom right corner of the screen.
10. Choose an option to verify your card. You can either process a verification code via SMS, or complete manually by calling your issuing bank. You can also choose to complete the verification later.
11. All done! The card has now been added to your Apple Pay Wallet and it can now be used for transactions.