Seat Reservation for Narita Airport Express

Using our JR Pass, we will be using the Narita Airport Express tomorrow afternoon, and been reading blogs that we have to make seat reservations! We will be traveling by Shinkansen from Shin-Osaka to Tokyo Station then take the Narita Airport Express. Now, can we make the reservations in Shin-Osaka, in Tokyo Station or in the internet? Thank you.

You can make reservations at all JR ticket offices in Japan, including the ones at Shin-Osaka and Tokyo stations.

It also seems that you can use the JR West online reservation system to make the reservation (assuming you do not use a rail pass). You will have to pick up the ticket before leaving Osaka:
https://e5489.jr-odekake.net/e5489/ibpc/CBTrainEntryExternalPC?LANG=en

If you use a rail pass, you should go to a ticket office.

Note: "Narita Airport Express" does not exist. The name is Narita Express.

So, you will leave Shin-Osaka on tomorrow May 7?

Shinagawa Station is more recommendable
for your transfer from Tokaido Shinkansen to Narita Express.
In Shinagawa Station,
the two platforms are closer to each other.
In Tokyo Station,
the two platforms are both horizontally and vertically.

For your last-minute Tokyo shopping, Tokyo Station sounds better.

Good news to you is,
all the airport-bound Narita Express trains scheduled on tomorrow May 7
currently have seats unreserved
at least for Shinagawa -- Tokyo -- Narita Airport (Terminal 1),
both in Ordinary Cars and in Green Cars.
Source: JR Cyber Station web site.

reading blogs that we have to make seat reservations

You may see someone using the seat which you have reserved.
For Narita Express,
there is a type of ticket with a seat not designated,
which allows the holder to use a reserved but vacant seat
until a passenger holding the corresponding reserved-seat ticket comes.

Thank you all for all your answers. Very helpful indeed. The Shinagawa sounds like a good option since I suffer from Knee Osteoarthritis and less walking is a God send. Thank you.