A saver day pass is an “all you can eat” pass for all public transport in Switzerland. Just go to a station, bus or tram stop, and just board the service you wish to take. Find yourself some seats, and enjoy the trip. There is no need to do anything in advance.
Trains are mass transit. There are usually enough seats for everyone. It does occasionally happen that more people board a train then there are seats, and then you just stand, usually only till the next station as there are always people getting off at each stop.