After creating a trip, tap the "+Add Lounge" button below any airport card to view the lounges that are available to you at that airport for free or for a one-time fee. Once you've determined the lounge you would like to visit, tap the "+" button next to the lounge name and the lounge will be added to the airport in your trip. At this time, only one lounge can be added to each airport in your trip.
If you would like to change the lounge you've added to an airport, tap the airport card, then tap the "+" button next to another lounge you would like to add. This will replace the existing lounge with the new lounge you selected.
If you would like to remove the lounge you added to an airport, tap the "Edit" button on the upper right corner. On the Edit Screen, tap the red circle next to the lounge you would like to remove and tap "Delete".