Just start it up and press the letter on your keyboard. You get 1 point when you zap a letter. You lose a point if you press a letter that isn't on the screen. You lose two points if the letter hits the bottom before you can zap it. The letters fall more frequently as you progress.
Press 1, 2, or 3 to change the level. Level 3 is pretty much impossible to keep a positive score once it hits the maximum letter fall rate.
You can use this game to help you learn proper hand positions on your keyboard - left hand fingers on "ASDF" and right hand fingers on "JKL;". Leave your hands in that position as you press the letters.