Grab a bar or rings and flip your body backwards so that your torso is horizontal and your chest is facing down.
The back is straight and horizontal.
One leg is extended and aligned with the rest of the body. The other leg is bend and tucked close to the chest.
The primary muscles work equally regardless of which leg you extend, so it is not necessary to count the exercise as unilateral: it is enough to alternate the legs with each set.
Form tips
Resist the temptation to round your upper back: this makes the exercise easier but also less effective. Keep your chest opened and look forward.