Sit on the floor, and carefully place a paper plate in between your feet (it's OK if it bends slightly to keep it in place).
Place your hands on the ground behind you for support.
Bend your elbows behind you slightly, and lean back to lift both of your legs up, making a "V" shape with your body. Keep your back flat and core engaged. If you think about lifting your chest, it will help prevent your spine from rounding.
Hold this position for one minute. If it's too hard, bend your knees slightly.