If you are talking about schedule planning, there are two sides to the coin.
Often, it is the airport people themselves who come knocking on the door. They will bring glossy brochures, slick sales presentations and powerpoint displays to convince airlines that they are worth a second look. Another venue is the air route convention, where airport managers and airline executives mingle and discuss these affairs.
Other times, the market is so obvious that the only hindrance is equipment type, such as CO
After preliminary discussion, the airline prepares a route analysis. This covers everything from market analysis, a study of the airport facilities, appropriate aircraft for the market, frequency, analysis of the competition, etc. Then the decision is made to open the route. Advertising, signing leases for terminal use, preparing air route data then follows. Then the route is announced, fares become available for pruchase...you know the rest.
A new life awaits you in the Off-World Colonies...