Aim with the mouse. Hold down the left mouse button and release when the error circle is at it's smallest. If you wait too long then the error circle quickly becomes big again. All rules are in the game, but here are the 501 rules anyway: score 501 points before your opponent to win a leg. You must finish on a double or the bull. So if you have 40 remaining, you must hit double top (double 20 - the outer segment of the twenty) to win the leg. There are hints to tell you what to aim for so you can easily find the best double.