Setup and Key Actions of Extended Side Angle Pose
Stand with your feet wide apart, facing the long edge of your mat. Keep your right foot parallel, or turn it in slightly. Pivot on your left heel so that your left toes point toward the short edge of your mat. Traditionally, this pose is practiced with the heel of the front foot lined up with the arch of the back foot, but if that feels unstable, you can walk your left (front) foot more toward the left side of your mat for a wider, more stable stance. Bend your front knee so that it stacks over your front heel (If your knee goes past your ankle, that's an indicator to widen your stance), and keep it pointing straight ahead. Err on the side of moving your knee toward the pinky-toe side of your foot while keeping the ball of your big toe grounded on the floor.
Bring your left fingertips to the floor (or a block) outside your left shin, and your right hand to your hip to start.
If you notice your front knee dropping in, lift your front heel off of the floor and press into the ball of your big toe as you draw your knee toward the pinky-toe side of your foot. Keep your knee pointing forward (not allowing it to drop in) as you lower your front heel toward the floor, grounding through the mound of your big toe and your outer heel. Press the top of your back thigh back, keeping your back leg straight and strong. Keep both legs working evenly, pressing your feet apart from each other as though you were stretching your mat in two.
You can keep your top hand on your hip, reach your arm straight up toward the sky, or turn your palm to face the front of the mat and stretch the arm alongside your ear (palm will face down). If your arm is extended alongside your ear, spin your top pinky down toward the floor to broaden your upper shoulder blades and release tension in your neck. Spin your belly and chest up toward the ceiling. Gaze straight ahead, or turn your whole head to look up.
Modifications of Extended Side Angle Pose
Place your front hand on a block outside of your front foot, or place your front forearm on your thigh (ardha parsvakonasana). Press your forearm into your thigh as if you were pushing it away from you to avoid collapsing in the shoulder joint.