You must be registered with a tesco.com account and a Clubcard account to be able to pay in-store with the Tesco Grocery & Clubcard app.
Just open the app, select 'Shop in-store' and then select 'Pay in-store'. With one scan of the payment barcode, you can pay, collect Clubcard points and get Clubcard Prices. You can also scan the payment barcode to release a 'Scan as you Shop' handset.