When you login to your Trezor device, after you’ve entered your PIN, you are presented with two choices: Open the Standard wallet or a Hidden wallet. If you click on Standard wallet, then there’s no Passphrase to enter. If you enter a Passphrase in the text field below Hidden wallet you either open your current Hidden wallet, if any, or create a new one, depending on your Passphrase. Therefore it’s important to remember any Passphrase you use, otherwise you won’t be able to access that current wallet.
Did you read the Wiki about Passphrase, which I linked to?
I don’t know of a step-by-step guide for it, but there are many posts in this forum if you search.
But, as I said, I don’t recommend using the Passphrase feature until you fully understand what it is and how it’s used. That goes for anything about Trezor, really, but Passphrase is a minefield to step into if you don’t know about what you’re doing and you may lose your funds completely if you step wrong.