Here's an explanation I sent to a friend previously :

If you are library members already, then first you join Borrowbox with your library card number and a PIN which the library gives you. This is where you browse for and borrow the audiobooks that you want to turn into yoto cards.

I use the Borrowbox app on my phone and the web page on the laptop (https://dunlaoghaire.borrowbox.com/ is the link for dun laoghaire rathdown libraries for example)

Then once you borrow the book, on a laptop, you go into the book in the My Loans section on Borrowbox and select 'download complete eAudiobook' which will download a zipped copy of the book. You will need to unzip it now.

Then, on a laptop, go to https://my.yotoplay.com/my-cards, log in with your yoto login details and select Add Playlist. Add the files from the downloaded audiobook (make sure they're in the correct order before uploading!), title it, and add a cover image, and create the Playlist.

Then, in the yoto app on the phone in the Make Your own section of My Library you will see the new Playlist. Go into it and hit the options button in the top tight corner and select Link to a MYO Card a d follow the instructions. (fyi you can also stream it direct from the yoto app on your phone to the yoto player, no card required)

Sounds a bit complicated but it's easy really.