Set a timetable choosing a route. Then head for the first stop, and once there you can set your sign on the bus (using the "92" icon), or using the IBIS.
What I used to do is drive to the termibus, to the first stop, and then select a timetable that started on that same stop, so I could just wait 5 minutes or advance 20, whatever the case. Then change the sign, and you're done.
Remember to set the sign again when you reach the last stop, so passengers know you're on your way back.
Good luck.