As a summary:
You'll need 7 planes, one for each day of the week. Each plane will fly one day of the route:
Monday to plane 1
Tuesday to plane 2
Wednesday to plane 3
You have to create each day of the route separately or create the first day then use the option "create a route like this for the next day". You can then rename all flights to the same flight number if necessary and they will appear in the world with the same number as if they were a typical 7-day a week flight.
Find more routes to fill up the rest of the free time on this schedule and that's it.
7-day scheduling is where the business management game turns into a puzzle game. You have to study routes and see what will fit on one set of 7-day scheduled 7 planes. Remember to not take off or land before 6-5 AM or beyond 24:00, and give sufficient turn around times between flights, and leave 5 hours somewhere in the schedule to perform weekly A-check maintenance.