Before performing the pose, determine two things about your support: its height and its distance from the wall. If you're stiffer, the support should be lower and placed farther from the wall; if you're more flexible, use a higher support that is closer to the wall. Your distance from the wall also depends on your height: if you're shorter move closer to the wall, if taller move farther from the wall. Experiment with the position of your support until you find the placement that works for you.
Start with your support about 5 to 6 inches away from the wall. Sit sideways on right end of the support, with your right side against the wall (left-handers can substitute "left" for "right" in these instructions). Exhale and, with one smooth movement, swing your legs up onto the wall and your shoulders and head lightly down onto the floor. The first few times you do this, you may ignominiously slide off the support and plop down with your buttocks on the floor. Don't get discouraged. Try lowering the support and/or moving it slightly further off the wall until you gain some facility with this movement, then move back closer to the wall.
Your sitting bones don't need to be right against the wall, but they should be "dripping" down into the space between the support and the wall. Check that the front of your torso gently arches from the pubis to the top of the shoulders. If the front of your torso seems flat, then you've probably slipped a bit off the support. Bend your knees, press your feet into the wall and lift your pelvis off the support a few inches, tuck the support a little higher up under your pelvis, then lower your pelvis onto the support again.
Lift and release the base of your skull away from the back of your neck and soften your throat. Don't push your chin against your sternum; instead let your sternum lift toward the chin. Take a small roll (made from a towel for example) under your neck if the cervical spine feels flat. Open your shoulder blades away from the spine and release your hands and arms out to your sides, palms up.
Keep your legs relatively firm, just enough to hold them vertically in place. Release the heads of the thigh bones and the weight of your belly deeply into your torso, toward the back of the pelvis. Soften your eyes and turn them down to look into your heart.
Stay in this pose anywhere from 5 to 15 minutes. Be sure not to twist off the support when coming out. Instead, slide off the support onto the floor before turning to the side. You can also bend your knees and push your feet against the wall to lift your pelvis off the support. Then slide the support to one side, lower your pelvis to the floor, and turn to the side. Stay on your side for a few breaths, and come up to sitting with an exhalation.