Hi
You will get a notification hung on your door the day before you check out normally there when we get up on our last full day. This will have the time for your bus as a printed voucher.
I tried to book mine 11 months out, and got an email telling me I had to wait until 30 days out. I can't find that policy anywhere online though, so I have no clue why... We'd paid in full at the time too.
I booked it online today for trip at end of October.
Last week I couldn't get the online reservation to work despite having my Disney reference number. Then I read that it sometimes works if you book the arrival and departure trips separately and that worked for me.
It is now showing as booked on our magic Band details.