From your Chaturanga Dandasana, untuck your toes while slightly sliding forward with your entire body. Press your feet into the ground.
Activate your legs. Imagine your tailbone lengthening to your heels. Feel your thighs hovering above the floor.
Pull your shoulders back and down. They are away from your ears.
To help you pull your shoulders back, keep a slight bend in your elbows. Imagine that your elbows and hands are pulling back. As you do this, feel your chest moving forward.
Imagine that you want to squeeze a pencil between your shoulder blades. Keep your neck long.
Tadaaaa!