You do not need to “book” a train route or validate your pass. Your pass is a “all you can eat” offer, where you can travel on all public transit as much as you like, at your own discretion.
I think you have concept wrong. The pass is not something that allows you to obtain free tickets. The pass is your ticket. It is all you need to travel.
So all you need to do is look up the schedule of the train you wish to take, go to the station, board and find a free seat. And when the conductor comes, show them your pass.