Reminder: if you want to modify an existing plan, which is in use or in production (status "Active" or "Completed"), first duplicate it (to do this, refer to the "Duplicate a transport plan" chapter of this article) and apply the modifications to the duplicate, then publish it.
To create a new service, go to your transport plan view (Resources > Transport plan) then click on the "Lines" tab and select the relevant line.
At the top right of the service list, click on "Add a run" and determine the run identifier in the pop-up window that opens.
Define your stop sequence (you'll find all the instructions you need in this article), then save your changes.
Then create a new route, using this guide. Generate the service timetable (dedicated article) and, if necessary, create a new calendar (click here to see how). Use this article to apply the appropriate schedules to the service runs.
Your new service is ready.