To transact on the Lightning Network you must use an external wallet that supports Lightning.
To make a Lightning payment you can start by pressing the 'Scan' button on the home screen. This will prompt you to scan or paste a valid Lightning invoice.
Once a valid invoice is scanned or pasted in, you can now choose whether to send from your BTC account or your GBP/EUR account by tapping the arrow in the middle of the screen. You can simply swipe the payment button right to confirm the transaction.
This will settle the payment instantly and for an extremely low fee compared to an on-chain transaction.