This is a question that comes up time and time again.
You have the pass. The pass serves as your ticket. As such you do not need to “book” train tickets. And because you do not need to book train tickets when you have a pass there is no way to do so.
So to travel eg. From Zug to Chur all you need to do is:
- Look up the schedule on sbb-tickets.cc or on the app
- Go to the station in time for your train (5 minutes is enough).
- Go to the platform your train leaves from, get on the train and find a seat.
When the conductor comes, show them your pass, and a valid ID (probably your passport in your case).
That is it. There is no need to book anything in advance. There is no need to do any formalities. Just find your train (or bus, tram, etc…) and get on.
For the Glacier Express you need a reservation in addition to either a ticket or pass, and you appear to have been able to obtain that.