Supta kurmasana is one of the four core postures of the 1st Ashtanga series. The others are marichyasana D, garbha pindasana and baddha konasana. They are core postures, because usually it takes some time till a yogi/yogini is able to do them. I needed 6 years for the above pose. No worries if this pose doesn't happen after the first attempts. Usually a teacher gives adjustments when a yogini is practicing this pose.
In the shala in Gokulam you're likely to be stopped when you cannot reach the fingers behind the back. It's not so important to cross the legs. Oh was I glad that I passed this obstacle when I was in Mysore a few years ago.
Our bodies are build differently. For some people supta kurmasana is easy, for others not. When the limbs are relatively long in comparison to the torso it might be easier.
There are 2 possible positions:
1. legs are crossed in front of the head or
2. legs are crossed behind the head/back.
For me it's easier to cross the legs behind back, see picture.
How I got into this pose: I used the sofa as a support. Leaning against it, I could take both legs behind the head. This is not so comfortable. I breathed deeply and relaxed. Then I moved my body forward the hands were stretched to the sides and pressed against the floor. I want to have a slow and safe landing. The head will come on the floor first. When the head is on the floor, I take my arms behind the back and hook the fingers. Looking at the picture I see, that it could be easier to bind a bit higher. It feels good to be in that pose. I can enjoy it.
How I approached this pose: To get the legs behind the head is easier when lying on the back and it's easier to have a soft floor. I often practiced this on the bed. This IS helpful. Try it first in the evening. Try to take the knees next to the body on the bed, then stretch the legs. Try to take the legs under your shoulders. On the bed all these attempts are fun (and easier). The next step is to do it on the floor........then one can try to do it while sitting......
My next step is to go away from the sofa........:) The pose is called dwi pada sirsasana.
Good luck, relax and let it happen........
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My book recommendation for today: Matthew Sweeney - Ashtanga yoga as it is.