Hi Karen, seat reservation can be done by either the staff at any JR Desk (this service is also free of charge) or simply by scanning the QR code on your Japan Rail Pass at any shinkansen ticket machine, found across all stations in Japan.

Feel free to have a look at this video tutorial on hwo the machine works. It really takes a minute or two to reserve seating and is free of charge as well: https://youtu.be/khpeDd5LDjU?si=cdWpjJC8SOfoMxJH

With kind regards, Jonathan.
Getjrpass Staff