Start on all fours with your hands directly under your shoulders and your knees directly under your hips. Engage your core, and find a neutral spine/flat back.
Bring your left knee and right elbow together under your torso, allowing your back to round slightly.
Without tilting your shoulders or hips to either side, straighten your right arm and left leg so they extend out from your body at shoulder and hip height. Avoid arching your back.
Bring your left knee and right elbow together again to begin the next rep.