First you need to understand that the Your Trip section of the App is simply a diary of your intentions. You can add or remove trains at will and until you actually add them to your pass they are just notes in a diary, you can delete them and add new ones at will.
So once you know the train you will be catching you can simply add it from the planner to your trip and then just before boarding add it to your pass by moving the slider. If this is the first journey of the day when you click “see ticket “ it creates a QR code for that day, which you present, along with any required reservation details, to a ticket inspector.
Any further trains you take that day need to be added to your trip and then into your pass in the same way before boarding. These use the same QR for that travel day.
If the train you want is not shown in the planner (normally identifiable by a train number) you can add it to your trip manually with as much info as possible - Always use scheduled times.