To do in-store purchases with Google Pay, follow these steps:
Look for the Google Pay symbol or contactless payment symbol at a contactless payment terminal near checkout.
Unlock your phone or launch Google Pay on your Wear OS watch.
Hold your phone or watch over the terminal and wait until you see a checkmark or hear the beep confirming the transaction.
When Google first unveiled Google Pay, it said you would be able to pay for goods by simply unlocking your Android phone like you normally do (which also enables Google Pay to authenticate your transaction), then placing it near a merchant's contactless terminal, and that's it. For purchases above $100, you'll need to put in your authentication. You don't even need to open an app to process a payment. It just lets you tap and go, and then you receive the confirmation/transaction details on your phone.