Yoga JANU SIRSASANA: Correct foot placement
Sit up straight with legs evenly extended in front. Bend the right leg at the knee and place the foot so that the heel is in the right groin and the front of the foot touches the left thigh. Turn the foot so that the bottom of the foot is facing upward and press the knee back to form an obtuse angle with the body. This position will be difficult at first; don't force it. Put a folded blanket under the knee and also under the hips. Gradually the knee will move farther back. Just keep the foot correctly positioned.
Yoga JANU SIRSASANA: Correct, perfect posture
Having positioned the foot and knee correctly, stretch the left leg out, keeping the leg firmly on the mat. Settle the heel firmly and stretch the toes up. (The heel should pull gently away from the ankle.) Now inhale and bend forward over the straight leg, catching the foot with both hands if possible. Beginners should bend only as far as they can without rounding the back. When this posture is done correctly and completely, the body will roll forward over the extended leg, absolutely flat from the tail bone to the head. Stay there breathing normally for as long as you can. Inhale, release the handhold, come up smoothly, straighten the bent leg and relax. Repeat on other side.
Yoga JANU SIRSASANA: Wrong posture
The heel is not positioned against its own thigh. The knee has not been pushed back as far as possible to form an obtuse angle. The back is humped and curved because the pelvis is jammed and unable to lift properly. Instead of a smooth, complete stretching of the spine, the lumbar is over-stretched and the rest of the spine constricted. The left leg is not flat on the floor.