Seat Reservations on Bullet train

We have two questions:

1. We are arriving in Tokoyo on 7th October staying for 3 days and then travelling initially to Kyoto on the 10th October. Once we have ordered our JR pass, how do we make seat reservations on the train?

2. We will be travelling onto Oaksa (15th), Hiroshima (17th), and Oita (18th via Fukuoka), how and when do we make our seat reservations?

many thanks
Mr Keith and Mrs Lyn Williams