Calisteniapp

On the ground face up

Contract your abs so that your upper back is lifted off the ground

Bend your legs bringing the knees to the chest

Extend the arms while protracting the scapulae so that the upper back is rounded

Now contract your abs further to raise your upper back higher and complete one repetition.

For a perfect technique, check that you do not have lumbar curvature and that the scapulae are protracted, at the same time that you contract your dorsal muscles to depress them.