Warm up to this pose with some gentle heart and shoulder openers; Gomukhasana (Cow-face Pose) and Ustrasana (Camel Pose) are a couple of my favorite preparatory postures for deep backbending. When you're ready, enter Viparita Dandasana from Urdhva Dhanurasana (Upward Bow Pose) by bringing the crown of the head to the floor. Release the forearms down one at a time and interlace the fingers behind the head. Advanced practitioners (a.k.a. those of us with too much time on our hands) may drop the legs into Viparita Dandasana from Sirsasana and, to exit the pose, kick the legs back up to Sirsasana from the backbend.
While in Viparita Dandasana, draw the shoulders back to slide the heart forward. Keep the elbows firmly pressed into the mat to prevent them from splaying apart or sliding up. At first, this pose may be practiced with the legs bent, but as proficiency is reached, the legs will gradually straighten. Press firmly into the heels and keep the inner and outer edges of the feet in contact with the mat. The feet may want to roll out because of the contraction of the glutes, which externally rotate the femurs. Counteract this tendency by grounding through the mounds of the big toes and consciously bringing an internal rotation to the legs. Ideally, the feet will be together, but tightness in the low back may require that the feet be hip-width apart which, for most of us, is only about 5-6 inches. (Your hips are not likely as wide as you perceive, especially since we're speaking strictly of bone structure here... just something to keep in mind.)
Engage Uddiyana bandha to support and elongate the low back. Breathe into your expanded, expansive chest and melt your heart forward by softening the space between the shoulderblades. Enjoy the strength and supplication of this pose.