Downward-facing dog: On your hands and knees, curl your toes under and lift your buns toward the ceiling. Try to be in a straight line from your hands to your buns, and another straight line from your buns to your heels. Keep your ears between your arms, and your shoulders pulled toward your waist, not near your ears. Pull your chest toward your knees.
Caution: Do not do this unless your back is strong and healthy.
Upward-facing Dog: From Down Dog, gently bring your hips down while continuing to support yourself on your extended arms. Look forward as your body lowers into a position that resembles Cobra, but your legs and torso are suspended in the air. Only your toes and hands are touching the mat. This requires some core strength.
If this feels totally OK on your back, move back and forth between the two positions, breathing with the movement.