We get this when we look at the aiming position. When we aim we put the target, the dart and the eye in one line. When we throw we should try to put the 3-lever system of the arm in a 2-dimensional plain. This reduces the margin of error by eliminating sideway movements of the arm. Thus it's easy to work out that we won't get happy with a frontal stance whith both feet touching the throwing line. Maybe this would be more comfortable, but we can't use it.
In an optimum stance the soulders and the board form a 90 degrees angle, and so does the line from the forward foot to the behind foot. But most people find this highly uncomfortable or even impossible. So we have to find a compromise.
The shoulder angle: It defines the stance. You should try to get close to the perfect 90 degrees, but not with an absolute sacrifice of throwing comfort. Proper angles start from approx. 50 to 80 degrees. The exact value here is your personal compromise.
The feet angle: The feet angle simply follows the shoulder angle, otherwise you will fall on the floor. Bang!
Weight distribution: Your weight will mainly rest on your forward foot (stance foot), while your behind foot (balance foot) will, well, just hold enough weight to perfectly balance your stance. Remember, no matter which weight distribution you use, your stance must be rock-solid. Don't jump with your forward foot, it must always stay flat on the floor.
Leaning forward: The more you lean forward the nearer you will get to the board (think that's why we do it, huh?), but the more physically demanding and unstable your throw will be. If you lean forward too much you will soon find yourself with serious back problems, especially when you combine it with sharp hip-bend. To keep yourself healthy use leaning forward wisely. Most pros have found a good compromize on this, and so should you. For advice just ask Bob Anderson about his back surgery.
Balance: Your balance leg is responsible for this work. Lifting it during the throw is not recommended, although it does by some pros, but these players have such a perfect balance they will still beat you with only one foot on the ground. Again, don't 'jump' when throwing - 'one foot on the floor' is not only a billiards rule. Highly recommended for darts players, too. Your balance foot shouldn't be flat on the ground (only okay for beginners), just the toes should touch it.
Torso: Your stance must guarantee that your body doesn't move at all when throwing. Remember the fixed shoulder joint of chapter 1. Don't arch your back, keep your spine straight. Your body must be strained, but your arm must stay relaxed.
As a summary, stance is the least difficult, but an important matter. Don't start with a frontal stance, this will disturb all other technical necessities, and you will find yourself struggling sooner or later.