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 (note that this limit does not apply 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.
Creating a base costs 50x the price of a gate at that airport (e.g., in an airport where each gate's rent is €100,000, creating a base will incur a €5,000,000 one-time charge.)
Once a base is established, you can return up to 3 gates immediately if you don't think you can fill up enough slots to keep all 4 gates. In addition, creating a base will now allow you to build a terminal.
Closing the Base
If you want to close a base, you will first have to close out all routes flown out of that base. The easiest way to accomplish that is under the View Routes page, selecting the base you want to close under the Show Routes From: drop-down menu, selecting all routes by click on each checkbox, then clicking on the Close Routes button.
Once that has been done, you can find the Close Base button on the View Gates or Rent Gates page.