1) If you have opened a new route, go to the scheduling page.
2) On the desired aircraft (the one you want to schedule the route to), click the calendar icon with a green + on it on the right of the scheduling view.
3) Check the "Add this route" button and click confirm.
4) Make sure to add maintenance checks to your aircraft (if you don't, the aviation authority will fine you). Click on the wrench icon near the calendar icon. Schedule the maintenance checks as appropriate.
5) Fill up your aircraft's schedule completely, except between the hours of 00:00 and 5:00. Remember, you must add your aircraft's turn-around time to the arrival time of the previous route to get the first available time for your next route.
6) The route will then be flown by the desired aircraft.