For cargo planes, it is the same as a passenger route. You have to open a cargo facility at the 2 airports. Cargo can make decent regular money, but passengers make more money.
Cargo planes are very profitable, especially the 707-320F. While cargo routes won't beat passenger routes for peak profit, they are stable all year and cheaper to run. Also consider the AN-24 a great option as you can easily fill it on any route.
