Hello Mark,
You will find the transit connection from the airport to Sants Station quite easy. The train departs from Terminal 2 at the airport. When you get to Terminal 2, follow the signs for RENFE, the Spanish train network. The train only goes in one direction from the platform as the airport is the terminus (with about three departures every hour). Here is the schedule:
http://rodalies.gencat.cat/es/horaris/. Once at the entrance to the trains, you can use the machine to purchase a T-10 card for about 10 euros. It is good for ten rides (can be used by multiple users) and can be used on all subways, buses, trains, trams in the city. You can take that train directly to Sants Station where you will need to purchase a ticket for your trip to Pamplona. Here is the schedule for today (08/04/2018):
https://www.renfe.com. Happy Walking!