There are two ways to add a card to your account:
Go to your Account Settings under 'My Ocado' and select 'Payment Details'. Click on the 'Add payment type' button and enter your card details.
You can also add a new card at Step 5 of the checkout. Simply, click on the 'Add a credit or debit card' link and enter your card details in the box provided. Then, when you move on to Step 6, you can store the details for future use.