Swiss Travel Pass

First class carriages have a big number 1 painted on the outside. Second class have a big number 2 on the outside. Because you have first class you can sit in either 1 or 2. There will be fewer people in your first class car than in second class. With a STP you just jump on any train any time anywhere.

Look at the train composition chart on the platform to see where the 1st class cars will stop.
Watch Rick's travel skills clip on train travel. From 1:30 on will show what I am referring to.
https://www.youtube.com/watch?v=3AxtbifeoBY

They "work" just like a normal ticket:
You go to the station, you get on a train (hopefully heading to your desired destination), sometime during the journey a uniformed person will come round and check everybody's tickets and passes, your train arrives at your desired destination, you get off and either get another train, or exit the station.

There is nothing extra you have to do, just travel as and when you like.

Here is a photo of an SBB coach, note the "2" by the door: http://werkstattatlas.info/images/stories/news/sbb/bpm51/Modernisierung_Bpm51_-_Foto_SBB_15.jpg
This is a 1st class one, note the "1" by the door, and the yellow stripe: https://modellbahnen.cadosch.org/cms/images/stories/Spur1_Wagen/kiss_sbb_apm61_bild1v.jpg
They come in other colour schemes and sizes as well.

In hindsight, 1st class is unnecessary, 2nd class is perfectly adequate.

If you have connections, and aren’t sure which train you will take, I find it helpful to take screen shots of the itineraries on the SBB site. Depending on your end destination. The trains and connections cities may not be consistent.