To play simply click on the flag that belongs to the country or organization named at the top of the screen.
Click again to move on to the next question.
Switch between Easy Mode and Hard Mode in the Options menu in the Menu Bar.
Easy Mode is a smaller group of only better known flags. All the flags in Easy are also included in Hard.
Hard Mode is the full set of all flags.
If you would like to reset your Statistics or your Best Streaks for both modes you can do that from the Options menu as well.