iPhone: Open the Wallet app, tap the «+» sign and add a card.

Apple Watch: Open the Apple Watch app on your iPhone, tap on «Wallet and Apple Pay», then on «Add credit or prepaid card».

iPad: Go to settings, open «Wallet and Apple Pay» and select «Add credit or prepaid card».

MacBook Pro with Touch ID: Go to system preferences, select «Wallet and Apple Pay» and click on the «+» sign to add a card.