Sit down on the ground with your legs straight in front of you. Put your hands next to your thighs and push yourself up (straight arms!), leaving your feet on the ground. Hold for uptp 60 seconds. If you cannot hold for 60 seconds, do smaller timed sets totalling 60 seconds (3 sets of 20 second holds, for example).
Focus on pushing the shoulder blades down ("depressing the scapulae", scapulae meaning shoulderblades).