It is horrible.
You have to go through each word and select each letter using the 'left' and 'right' button, then choosing it by clicking with both buttons.
This can take quite some time until you have entered your 24 words.
It really is. I had to go through it about a month ago when an update suddenly wiped my Nano S clean. It takes 5-10 minutes to get it back up and running again. My main concern was that I didn't know why the device wiped itself clean since I never experienced it before. It had me worried until I restored my account.