My kit arrived. What do I do with it?

Tap the stamp with your own phone once. A page opens: if you are not signed in it says "This tag isn't registered yet. Sign in to register it to your store." — sign in with your dashboard account and you come straight back. Then confirm "Register this tag to {your shop}?" with the Register button. After that, the stamp belongs to your shop and every tap earns stamps for your customers.

You only do this once per stamp. Once it is done the register prompt disappears from your dashboard, and Settings shows "NFC tag — Active since {date}".