Nozomi Trains

Hello. We are travelling to Japan in November. Before I purchase our pass, I would like to ask a few questions.
At one point we travel from Naoshima to Tokyo with a stop at Himeji. We would like to use the Nozomi line. I understand that as of October, this line will be included in the JR Pass but we'll have to pay an additional fee.
Can you tell me what is the additional fee we should expect to pay or that trip? (Okayama-Himeji-Tokyo)?
Where do we pay this extra? Ahead of time? When booking the seats?
Should we purchase the Green JR PASS, do we have to add a higher fee for the Nozomi trip?
Thank you. MEC