Setup and Key Actions
Begin in tadasana (mountain pose), with feet parallel and four to six inches apart (the distance of about two fists). Then shift your weight onto your left foot as you bend your right knee, and place the sole of your right foot below or above your standing-leg knee.
Remember that this pose isn't about how wide you can open your knee out to the side. Rather than trying to crank open your lifted-leg knee as much as you can, begin by focusing instead on aligning your pelvis. From your initial setup in tree pose—balancing on your left leg with your right knee bent, with your right foot pressing against your standing-leg thigh or calf—lift the sole of your right foot from your leg so that only your heel presses against it. Then pivot your right heel on your thigh (or calf), turning your toes to point more toward the front surface of your left leg, and your right knee to point out at only about a 45-degree angle (as opposed to directly, or nearly directly, out to the side where it may have been before). This will take a little of the external rotation out of your right hip, helping to re-align and stabilize your pelvis.
Keep your two frontal "hip points" (often also referred to as "pelvic points," technically known as your “anterior superior iliac spines,” or ASIS) pointing forward. With both of your hip points remaining straight forward (not rolling open to the right as your thigh turns out), pivot on your right heel as you externally rotate your right thigh. You'll likely feel a bit of engagement in the right side of your butt as you do this. Externally rotate only as much as you can without taking your hip points with you.
Press your foot against your leg, and your leg against your foot.
Un-grip the toes of your standing leg foot, and rest them on the mat (or, if you tend to roll to the inner edge of your standing foot, you may find it helpful to initially lift your toes away from the floor to give a little lift to your inner arch). Let the three "corners" of your feet (the mound below the big toe, the mound below the pinky toe, and the center of the heel) ground into the floor beneath you.
Keep your hands at your heart center, rest them on your hips, or extend them overhead.
Practice with your back facing a wall, or with your right hand on a wall for more stability.