First, you download an "UnoJoy", than you put code in Arduino (in the folder of UnoJoy, called UnoJoyArduinoSample).
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Step 2
Than put arduino uno r3 in "DF mode" (you make this by touching 2 pins on the 6 pin connector, closer to USB B connector, (where you put an USB cable)), than start "TurnIntoAJoystick".
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Step 3
Than, you connect your buttons to pins 2-13 and analog pins (in the folder, there are written coresponding buttons to pins, or try it with metod - try and error, like me :D