When getting off the Narita Express (NEX) you will be in an underground area like this. Go up the lift and follow the signs towards the Hokuriku Shinkansen, tracks 20-23. See Below
Keep following the signs towards the Hokuriku Shinkansen toward Tohoku, Yamagata, Akita, Hokkaido, Joetsu, and Hokuriku (via Nagano). This Shinkansen is always green on the signs and departs from platforms 20-23. Even if you do not come from the Narita Express you will see signs for the Hokuriku Shinkansen throughout Tokyo Station.
Purchase your ticket at any Ticket Office. Some are in closed-off spaces and some are in the open like this. If you reserve seats, you will need to go on a specific Shinkansen, if not you can go on any Shinkansen. You may also want to verify your JR Pass at Tokyo Station. The ticket office will direct you to the JR Pass office. Please note that some trains are reserved seats only. The unreserved seating is typically from cars 1-5.
The ticket will not say the platform of your Shinkansen. You must look at the large boards (much like an airport) to find the track number.
Enter the ticket booth (putting your ticket in the slot) and find the platform of your train.
Note: All trains going through Nagano stop at Nagano Station. However, not all trains stop at Iiyama Station. Please check on Google Maps or with a station attendant to make sure your train stops at Iiyama Station when traveling there.