Apple Pay setup is very simple
Add additional cards on iPhone 6 within the Passbook app. Just tap the plus sign and follow the onscreen instructions. On iPad Air 2 and iPad mini 3, start by opening Settings, tap "Passbook & Apple Pay", then tap "Add Credit or Debit Card". For Apple Watch, open the Apple Watch app on your iPhone, tap "Passbook & Apple Pay," then tap "Add Credit or Debit Card." The first card you add will be your default payment card, but you can always choose a different card when paying or select a new default.