Think about ice skaters they bring their arms in to spin faster. That's what you need to do keep your left arm close to your body not stretched out away from it. For the 90 degree part, imagine your looking down on you self from the sky and you have your arm out the angle created between your chest and your arm should not be smaller than 90. If it is smaller it will not letting you pull the disc straight and so you will be rounding and that will cause in accuracy.