Lie on your back.
Contract your abs so that your upper back lifts off the ground.
Bend one of your legs, bringing your knee to your chest, while keeping the other leg extended.
Extend your arms while protracting your scapulae, so that your upper back is rounded.
For perfect technique, check that you have no lumbar curvature and that the scapulae are protracted, while at the same time contracting the back muscles to depress them.