If you don't want to trust a third party, just get Bitcoin Core to a computer without any internet connection, generate your keys and use it that way. You can sign a transaction on the computer and push it to blockchain on another one.
Can you share a link to a tutorial?
The whole process
looks a bit complicated for Bitcoin Core. It's more common for its users to keep the wallet online in order to have all the (latest) blocks downloaded. Consider choosing Electrum instead,
it will be much easier for you. No commands involved.