Setup and Key Actions
From tadasana (mountain pose), bend your left knee so that your knee points toward the ground and your heel moves toward your seat. Reach your left arm back and hold the inside (big-toe side) of your left foot with your left hand (holding the inside makes it easier to maintain external rotation in your upper arm, preventing your shoulder from rolling forward).
Begin with your standing and bent-leg knees next to each other. Spread your left toes, and press your left hand and foot against each other to create some resistance. Kick your left foot against your hand to lift the foot up. Try to lift your foot up faster than your body comes forward. Broaden your collarbones and lift your sternum. Draw the back-leg side of your belly toward the front-leg side of your belly to prevent the hips from significantly rolling open.
You can extend your right arm forward, alongside your ear, with the palm facing in, or even bring your right hand to your heart to encourage the chest to broaden and lift, bringing more of the backbend into the thoracic spine (middle/upper back).
Natarajasana With a Strap
To work toward "catching" your back foot with both hands reaching overhead (a variation in which both of your elbows will point up, requiring you to fully lengthen through your triceps), loop a strap around the sole of your left foot. Hold the loop with your left hand (palm facing up), and externally rotate your left upper arm. Keep the external rotation as you spin your left elbow up toward the ceiling, and your torso toward the short edge of your mat. Then, reach your right hand up and back, bending your elbow to catch hold of the strap with your right hand as well. Keep your spine long, and keep your torso facing forward. Walk your hands down the strap as you continue broadening your collarbones and lifting your chest. As your chest lifts, move your head back to follow, keeping the back of your neck long as you explore the backbend.
If you're able to bring your hands to meet your foot using the strap, you may be ready to explore the pose without the strap.
Natarajasana Without a Strap
From natarajasana prep: Reach your right hand across your body and catch hold of the big-toe side of your left foot to help keep it in place while you change your grip with your left hand. Keep your left toes active and your left knee hugging in toward center (not splaying open). Then externally rotate your left arm (the way you did with the strap), and wrap your thumb around the pinky-toe side of your left foot, and your fingers across the top of your left foot, holding onto the big-toe side of the foot. Once you have a good, solid grip with your left hand, release your right hand and spin your left elbow up toward the ceiling and your torso to face the short edge of your mat. From here, reach your right arm up and back, bending your elbow to hold onto your left foot with your right hand as well. Again, instead of trying to touch your foot to your head, explore an even backbend: broaden your collarbones, lift your chest, and keep your neck long as your head moves back. Here too, stay here only as long as you can comfortably breathe before returning to your starting position and repeating on the other side.
If you can't catch hold of your foot with your hand in the prep, loop a strap around the arch of your foot, and hold on to the loop.