After entering the UNIQUE_KEY, select “Unlock Bootloader” on your phone with the volume down button and confirm with the power button.
The next point that confused me for a second was this:
But you just have to use the volume buttons and select recovery on the smartphone screen from the menu you’re in. I just didn’t realize it was a menu at first…
All in all, everything worked without any problems and took maybe 30 minutes in total.