Janu Sirsasana (Head to Knee or Head of Knee pose) is a seated forward bend which teaches the actions of all the other seated forward bends and prepares us for the "king" of forward bends, Pashimottanasana (Intense stretch of the West).
Before we can bring our heads down in seated forward bends, we must learn to lengthen and extend our front body so the chest does not collapse.
As with the other seated poses, take blanket height to keep the pelvis tilted up as necessary.
Start with you hands by the sides of your hips and use the arms to lift the chest.
The straight leg is like Dandasana. The bent leg is somewhat like Baddha Konasana and Virasana. Be sure to keep any twists out of the bent knee to protect your knee.
If your knee hurts in this pose, sit higher up on more support.
Now raise your arms up AND keep your hips and legs grounded.
Then, extend forward and grab the sides of your straight leg foot. If you cannot reach, grab your shin or use a belt.
Elongate the front of the spine from the navel to the chest towards the sky AND keep your straight leg grounded into the floor.