The easiest way is at a ticket office in Switzerland. Depending on when you plan to travel it is often unnecessary to reserve in any event. We’ll give you more guidance if you give us FULL travel details 

Asked and answered in your last thread on this topic.

You normally don't need to make any reservations for that train. I travelled from Spiez to Montreux both during Easter and in the beginning of August. Both times the train was rather empty.