In this version of Virabhadrasana III (Warrior III), you'll use blocks to support your upper body and a wall to take some of the weight off your lifted leg, helping you strengthen and stabilize your legs, hips, and sacrum.
Start in Tadasana (Mountain Pose), with your back facing a wall that's about a leg's distance away. Have two blocks handy. Fold forward into Uttanasana (Standing Forward Bend), lift your left leg behind you, and press your left foot against the wall at hip height so that it's parallel to the floor. Inhale as you lift your spine away from the floor and place one block under each hand. See that your hands are beneath your shoulders.
Build your pose from the ground up. Spring the arch of your foot upward. Then press the outer edge of your standing leg inward toward your inner leg. Next, imagine zipping up a long zipper from your inner ankle to your inner groin to help you lengthen the inner shaft of your leg. Lastly, shave the outer edge of your right hip back toward the wall behind you. Stay here for a few breaths, making sure that the whole leg works evenly; no part of it should feel slack.
Bring your attention to your upper body. Slide the front of your spine, from just below your navel, toward your heart. Do this without hardening your belly or sucking it back and up. Simultaneously, elongate the two sides of your tailbone away from your lumbar, toward the wall behind you.