To assign a card to a trip, or change the trip a card is assigned to, navigate to the cards details screen and scroll to the "Other Information" section to find the "Trip" heading. On tapping into the trip sub-section you can select one of your existing trips or create a new trip to assign the card to.
Look for the 'Trip' heading:
Select the trip you want the booking assigned to:
Your card will now be linked to your selected booking: