I bought the Iberic Rail Pass to go to Spain and Portugal on my recent trip a couple of weeks ago.

All you need to do is to reserve your seats at the train station. You'll need to pay a supplement if you're taking the AVE or the overnight train. I paid 3500pts for Preference class return to Sevilla on the AVE.

The booking counter will validate your pass the first time you use it. All you need to do is to enter the date of your travels. I didn't enter the 1st one so I could have had an extra ride if I wanted to.