Clairebear commented that it's already hard to lift the body up with crossed legs. And it is demanding. It already requires core strength. One must be able to keep the legs close to the body. Muscles need to be engaged that we usually do not use.
There is always a tiny step less demanding. If it's not possible to lift the body up, it can be an approach simply to keep the legs as close to the body as possible. First one might like to hold them with the arms. The next step is to keep the legs close to the body without support of the arms.
Bandhas need to be engaged.
Then one can try again to lift up the body with an inhaling. Abdomen need to be engaged. With the next exhaling one can lower the body to the floor again. Shoulders move down. Repetition. After a while it should be possible to hold the pose for 2 breaths, then for 3 breaths.....
Another exercise is to have one leg on the arm the other one is kept as close to the body as possible. The one hand needs to be pressed against the floor very strongly in order not to sink in on one side. My opinion is that one needs also a lot of strength in the arms in order to manage it to jump back. I try to hold the position for several breath. It can be good to hold it for 15 breaths, too.
From the position below I try to float back. So far the one feet touches the floor. Repetition is necessary. Nevertheless I think this could be an exercise to approach the correct vinyasa.