Quite easy :) But remember to use the testnet for that!
- 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 testnet tokens to the displayed address with the data hash attached to the transaction. You can find an example here: https://github.com/F-Node-Karlsruhe/ghost-iota-pay/blob/main/test_wallet.py
- Wait till the payment is recognized by the gateway and the article will show up automatically.
You can get testnet tokens for testing here: https://faucet.tanglekit.de