Backing up a car involves turning the wheel in the direction you want the rear of the car to turn. Putting your left hand at the top of the steering wheel and putting your right arm on the passenger seat gives you the right posture for safe reversing.
Putting on the brake is generally required to shift from park to reverse. While the brake is engaged, slide your left hand to the top of the steering wheel, pivot your hips slightly to the right, put your right arm across the back of the passenger seat and look back over your right shoulder. Control speed solely with the use of the brake pedal, if possible, to maintain a safe speed.