To choose the card you want to pay with, tap the home button and tap the card you want.
You can see all your cards by tapping the grid view button on the top right
You can choose to use either the grid or carousel view by tapping the button on the top right. (If you don't have this option, please upgrade to Curve 2.0!)
There'll be a short animation of the Curve logo 'loading' and then you know the new card is selected. To check which card is selected, look for the Curve logo 'watermark' overlaid on the selected card image in the app.
It will continue with this card until you select a different one. You can then make as many payments as you like without switching cards. You can choose a card at any time up until the time of payment (unlike Apple Pay, which requires you to tap the card in the app at the point of purchase).