Trips are created by tapping on the + button from the Trips page (default startup page). If you are using any of the automated start/stop features (Bluetooth or Places), trips will automatically start recording for you once the conditions are met. You won't need to tap on + or fill in any values yourself.
There are also several shortcuts in creating a new trip. This can be from Siri Shortcuts, using the Travel Logs Widget, Apple Watch App or through 3D Touch (Force touch on the app icon).
To edit or stop a trip, tap on the in-progress trip.
To ensure that trips are recorded with minimal effort, make sure you have Pre-fill start odometer, Auto-fill locations, Track distance traveled in realtime enabled in the application settings.