Janu Sirsasana C is one of the most challenging poses of Primary Series. It's very confusing and scary to new students. This video talks about some techniques for approaching this pose.
Remember for this one, its the hip joint and the ankle that rotate - not your knee. Your knee is open about 75 degrees to the side - and there should be no pressure on your knee. It should all originate from the HIP JOINT - but people with tight hips tent to recruit flexibility from the knee here. Additionally, it is one of the deepest flexions of the knee in primary series.
I use a bench or windowsill to teach this pose to new students. This helps the student understand what the foot and hip are doing in the pose - because when the student is standing its much easier to open the hips. It also gives the student a lot more space to work with.
Want to learn more about opening your hips? Check out my online course Be Hippy!