Lie facedown on your forearms with the elbows aligned with your shoulders, and the feet about hip-width apart (widening the feet will create a more stable base to make it easier, and closing the width will make the exercise more challenging).

Tilt the hips into a posterior pelvic tilt (pointing the tailbone down towards the ground). If it is no longer possible to maintain the posterior pelvic tilt before the completion of the desired time, end the set.

Lift the right elbow and place that hand behind your back, and lift the left leg off the ground. 

Repeat with the other arm.

During the entire set, tighten the core and glutes as much as possible.