Hi, seat reservation can be done for free with a JR Pass by either scanning the QR code on your JR Pass at any Shinkansen ticket machine (found at all JR stations) or by visiting any JR desk and get help to book it by JR Staff. Both options are free with a Japan Rail Pass.
Seat reservation is not a requirement, but worth doing since it’s for free with a JR Pass.
However, all trains are equipped with a couple of wagons without seat reservation which you may use without reservation and just can jump on.
Here is a video on how to do it via ticket machines if you’d like to try yourself: https://youtu.be/khpeDd5LDjU?si=4Lv94aw0Y_IclV98
Hope it was of help and have a wonderful stay in Japan! 😊