Create Base
Before you start
In order to create a base you have to make sure that the airport you want to serve is on the same continent as where you started the game. So if you started in Africa, you stay in Africa. This is an anti-competitive measure to ensure the very large airlines cannot flood all markets and to make the game fairer to everyone else. You also have to make sure you have the correct number of gates at the airport, the minimum is 4 gates to start off with. Of course you can rent many more before creating a base but it becomes very expensive at that point. There is also an restriction on the number of bases, again this is to stop very large airlines taking over an entire continent and squeezing the smaller airlines out. The limit is 6 bases. When hubs come online these will be limited to 4, so in total the maximum number of airports that an airline can fly from will be 10(not limit to Alliance Routes).
Creating the Base
If you have satisfied these requirements and you are ready to create a base, then go to either Rent Gates or View Gates, here you can see on the top right the options to create or close a base. Click on Create Base, then select the airport you have rented the 4 gates at (You did remember to rent those gates first and the airport concerned is on the same continent right?) and finally click Create Base.