Setup and Key Actions
Sit tall, knees bent, feet flat on the floor. Slide your left leg under your right and bring your left heel to the outside of your right hip. Bring your right foot around to the outside of your left hip. If the pose feels inaccessible from a seated position, you can instead start on all fours: Cross your left knee behind your right, and separate your feet so that you can sit back between them.
Once you're seated with legs crossed, press your fingertips into the floor, pick up your hips, and scoot your seat toward the bottom-leg side to help level your sit bones. Spread your toes and make sure that the pinky-toe sides of your feet are in contact with the floor. Then, since your right leg is on top, inhale and lift your left arm up, bend your left elbow and let your left hand rest at the middle of your upper back. Internally rotate your right arm so that your thumb faces in toward you, and reach back and around, bringing the back of your right hand to rest on your back. Perhaps you're able to clasp your hands here. If not, hold a strap between your hands. Lengthen your spine, and breathe into your back ribs (avoid jutting your bottom ribs forward). Lengthen especially through the bottom-arm side of the body, and draw the head of the bottom-arm bone back (it won't go all the way back since the arm is in internal rotation—you're just seeking balance). Gently press the center of the back of your head into the part of your your top arm it's touching (if any).
If you're having trouble stacking your knees, place the sole of your top foot flat on the floor on either side of your bottom knee (as in the seated twist ardha matsyendrasana, aka half lord of the fishes pose). If you're having trouble clasping your hands, place a strap between them.