Setup and Key Actions
Begin seated with your legs extended in dandasana (staff pose). Relax your right leg, bend your right knee, and place the ball of your right foot at the inner thigh of your straight left leg. Instead of pointing both hips forward, like a seated tree pose, allow your hips to open (think upavistha konasana with one knee bent). Spread the toes of both feet to activate the muscles of your legs. Use your hands to move the flesh of your bent-leg thigh in, back, and apart. (You might find it helpful to do this to your straight-leg thigh too—especially if your lower back tends to round, but generally, the bent leg will need that action a little more.)
Sit upright, and press your fingertips into the floor to activate the muscles of your lower belly. Bring your right hand to rest lightly on your right knee cap. Turn the right side of your belly toward the left to rotate your torso to face your straight leg. Keep lengthening your spine and turning from your belly as you extend out through the head of the right knee in the opposite direction. If you can maintain length in your spine, bring your right hand to your left outer thigh, shin, ankle, or foot, and walk the left hand more forward. Remain in extension, or come into the forward bend by lengthening forward over the straight leg.
This pose's name is a bit misleading—you don't want to actually bring your head to your knee, but to extend your torso forward over the bent leg, leading with your heart, and then rounding evenly through your spine. (Some yogis even prefer to refer to this asana as “head of the knee pose” instead of “head to knee pose” as a reminder of the importance of extending out through the head of the bent-leg knee while turning the belly in the opposite direction). If you can comfortably hold your foot, clasp your opposite wrist around the sole of the foot, and bend your elbows out to the sides.
If you find that you're collapsing in your lower back and unable to sit up straight, sit on a folded blanket. If you tend to lock your straight-leg knee, or if your straight-leg knee is dropping in, place a rolled blanket or yoga mat underneath your knee and gently press into the blanket to activate your inner quadriceps.