On the floor, face up.
Engage your abs so that your upper back lifts off the ground.
Bend your legs, bringing your knees up to your chest.
Extend your arms while protracting your scapulae, so that your upper back is rounded.
Now contract your abs even more to raise your back higher and complete one repetition.
For perfect technique, check that your lumbar is not curved and that your scapulae are protracted, while simultaneously contracting your back muscles to depress them.