After downloading the app, tap on the + Add to wallet button and choose Payment Card. Insert the card details and save (bank authorization is required). Once the card is added, enable NFC and open Google Wallet when you’re ready to pay.
Make sure the correct card is selected, and tap the device on the contactless card reader. Confirm the purchase with your passcode or fingerprint if required. The wallet will show you a tick symbol when the transaction is complete.