1. Create a small airport in the two largest, and furthest away cities on the map. EDIT: Larger cities have more passengers / mail. You're also paid per mile, so the further away it is, the more profit you make.

2. Buy an aircraft, and send them to deliver passengers/mail between the two large and far away cities.

3. As long as this aircraft doesn't crash, you're now set to mess around at your own pace. It delivers a solid amount of cash. If the aircraft crashes (bad luck), either restart the game, or take out a loan for a 2nd aircraft.