A number will be written literally (in words) on the bottom of the screen.
Shoot the airplane that has that number by clicking it.
Note that the airplanes descend slowly - don't let them hit the Eagle!
As you progress through the game, the number of airplanes and their speed will increase gradually.
When you reach the second level of the game it will become more challenging - the numbers will be very much alike (such as: 8465, 8645, 8654 etc.) so pay close attention.