Put your fingers on 2 and 3 on your keyboard. These are your firing keys! Aim with your mouse.
When you fire, you subtract off those numbers. Your goal is to get the target number down to EXACTLY zero. If you don't, well, you'll get yourself in trouble fast!
For the next stages, you'll be told to move your fingers to higher numbers.
TIP: This game is fast... You may want to study the numbers for the stages so you can think faster! What numbers do you think will come up for the different stages? You can figure this out and study!