is quite simple, use the arrow keys to move and Z to shoot. But the trick to getting a really good score is to sit back and do nothing. The longer you wait, the more missiles your ship locks on to the enemies. Any movement, or firing, resets the counter and fires off any missiles that have been locked. Your score for any locked targets is then multiplied by the total number of missiles fired at that moment.
Thus the key is to wait as long as possible, in order to get more points. Your ship’s hitbox is the point in the middle of it, so while many of the enemies shoot at you they often miss by a little bit. This should tempt you to wait just a little bit longer. The game does get quite hard, and is perhaps a little unforgiving when you die.