Start by standing straight with your legs together. Preferably a weight in your hands, for example a barbell, dumbbell, kettlebell or any other weight.
Begin the movement by tucking your chin into your chest. Then, slowly continue to round your back one vertebrae at a time. As you curl, feel the weight "pull" you lower towards the floor. Stay balanced on the balls of your feet to keep from leaning back.
Your entire spine should have one uniform curve to it at the bottom. Focus on pulling your belly button into your spine. Remember to breathe normally throughout the movement.
Reverse the movement by uncurling the spine beginning with the low back first. Keep your chin tucked into your chest the whole time. When you are standing up all the way, finally extend your neck straight.