Start off on your hands and knees with your hands set underneath your shoulders. Then walk your knees back out behind you, tuck your toes and pick your knees up off the floor into Plank position.
Make your body as rigid as you can from your feet to your head to keep it in a straight “plank-like” shape side on. Do this by tensing your quads to lift your knee caps up your thighs, squeezing your glutes together, bracing your core and drawing your belly button up away from the floor, whilst at the same time pressing down into the floor with your hands.
Whilst maintaining this strong plank position, shift your weight slightly to your right and then lift up your left arm up to shoulder height so that it is reaching out in front of you. Aim to keep your hips and shoulders level whilst balancing on one arm. Then return your left hand to the floor below your shoulder. Shift your weight over to the left and then raise your right arm up and out in front of you.
Keep alternating arms slowly, and with control for the duration of the exercise.