Quite easy :)
- Go to the article with costs https://blog.f-node.de/how-to-pay-per-content-with-ghost/ and click the "Buy with IOTA" button.
- Transfer the demanded amount of IOTA tokens to the displayed address using your Firefly wallet by clicking on "Firefly Wallet".
- Wait till the payment is recognized by the gateway and the article will show up automatically a few seconds after the payment.
WARNING: The video below shows an outdated version of the gateway. An updated tutorial video will be available soon.