It's dead easy, just use the fc barcelona . com website. Select 'Tickets' then the game in question and then choose your seats. Tickets for the Celtic game are already available for purchase, but buy them sooner rather than later to ensure you get seats next to each other.
I've used it myself a few times as I live here in Barcelona. The prices are the same on the website as in the tourist information and Barca shops around town. In Spain you can then just print out the tickets immediately after purchase yourself, and if buying from abroad (if printing is not an available option) just select to collect from the ground on the day of the match. I know people who have also done this with no problems.
Enjoy and good luck!
This message was last edited by danb on 22/10/2013.