This is also sometimes called back star.
Enter the pose from sitting. When doing it for the first time, you can sort of shuffle into position: you want to move your pelvis forwards so that the hoop is basically on your lower back. You can then straighten both legs. Alternatively, a more elegant way to get in is to striaghten one leg towards the opposite side of the hoop, and then slide the foot around the hoop into place, and at the end you straighten the other one too.
This pose should feel relatively stable, so you can release your hands. If it doesn’t, try to adjust it slightly. The bar is typically right above your bum, but it varies person by person.
Ideally the bar should be on your upper thighs. If your legs aren’t flexible enough, you might want to work on your pancake splits.
From here, you can try going into back balance, but for many people (including me), the balance point is not exactly the same. Another way to come out is through bird’s nest.
One way of getting into the pose is shown here: