I've not made it very obvious - sorry! If you're using the shredder it is expecting to get a signal from a USB device (it expects to get a keypress of \ and | at the same time). I'm thinking to add instructions on how to build one before it's out of beta.
You probably want to start a new file and select "bin" at the beginning. Maybe I should make the bin default as most people aren't going to have a custom-built shredder 😅
Thanks for playing and for the feedback :)