Installing RetroPie to Raspberry Pi 3 with wireless Xbox360 controllers
Syncing and configuring Xbox 360 controller
6. When RetroPie has started you will be greeted with this message. You have to hold A button from your controller, but if it doesn't work, then you have to sync your controller and receiver. (You might notice that RetroPie's resolution / aspect ratio isn't filling your whole screen. We are fixing this later.)
7. Press sync button from receiver and then from the controller.
8. When your controller is synced. Flashing light should be stop and be in place.
9. Next step is to define buttons. This is annoying step if you press wrong button as i have pressed in this image. You can't go back if you press wrong button so you have to go through all the buttons and then do it again, or you can just reboot Raspberry. If there is buttons that you don't want / need to define, you can skip defining by holding any button that is already defined. So if you have defined all D-Pad buttons and want to skip defining Start button, you could skip it by holding any D-pad direction from controller. After defining buttons, press Ok.