Firstly, if your new please read this short tutorial.
If not then I recommend you skip this Part.
First when you load into a game of discover the world you automatically start in Germany.
Drag your mouse from one airport to another to start a route.
Click on a Airport to view its contents. It will have a pop-up that shows its capacity and lvl etc.
You can Upgrade the Airport by clicking on the blue circle with cash in it.
The routes button on the bottom of the menu simply shows the routes.
That pretty much it for Basic Controls. Comment if I missed anything because I'm in a rush to finish this.

Before We Begin This Strategy, You have to understand that most people want to go to the capitals in this game. For example Berlin will most likely fill up before Frankfurt because Berlin is the capital, Unlike Frankfurt.
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Note: Connect ALL airports in the country to its capital.
First, When you start you will have some close calls. Berlin May fill up to fast. To prevent this Connect one short and one long route to Berlin. You will have one airport unconnected. I suggest you save to get enough to connect that airport to Berlin. If you can't connect the airport to the nearest airport that is connected to Berlin. Once you have enough to connect it to the capital sell the route to the nearest airport that is connected to Berlin and place a route from the airport to the capital. then just save for your needed country to get. When you get your other country immediately save to connect the capital to Berlin. As you progress you will repeat these steps but make sure every country's capital is connected to ATLEAST 2 other Country's capitals.
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ALSOOOOOOOOOOO if you see a plane in red I usually upgrade it and if the plane is lvl 5 and red I get another plane to lvl 3 or 4.
If a capital is red look for routes with dark orange or red routes and upgrade the plane.
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That was my first guide so comment if anything is unclear or ur confused about and I'll try to fix.