When adding/editing a trip, you need to make sure that you've included all the stops that that trip passes through in the stop sequence.
Once you click directly on the name of the trip to enter the mapping page you go to the tab Stop sequence. And here is where the magic happens! You will start 'connecting' the stops - you may use existing stops that were already created or create new ones.
For existing stops, you can use the stop search bar to locate the first stop of the trip (or move around the map until you find it).
If a stop does not appear on the map you should create the stop on the map before you add it to the trip.
To start connecting them, you must Left-click on the correct blue stop icon to make the stop the first of the trip.
Continue searching for the stops in the correct order and left-clicking on them to create the full stop sequence.
Pay attention and check that you are connecting the stops on the correct side of the street!
Click Save to complete the trip.