The boule should rest comfortably in the hand with the fingers held as close as possible to each other. The thumb should be very close to or touching the side of the index finger.
When you throw the boule both feet must be fully in the throwing circle. The feet do not have to be together but must be on the ground until the thrown boule has landed.
The most widely used method of throwing is underarm with the palm facing down. There is no rule to say you must throw this way, but it is better for control as it imparts backspin.