Practically, I would use some of the skills from your front squats and apply them here:
- aim to slot your bum between your ankles;
- the brace on the bar should pull with your pecs against your lats and traps/rhombs (imagine bending the bar in half to bring the ends together in front of you.... Meaning:
- the bar should start, travel and end up over the crown of your head (I.e. Back of the top if your head);
- keep your thorax as upright as possible at all times, but crucially at the top and bottom of the movement. You will understand why once you put some load over your head.
Whilst keeping your adductors warm, do some work on your calves, too, to assist with that there ankle mobility.
Good work, however! Once you get your balance sorted, you obviously have more than enough strength to handle a lot of weight in this... I would honestly just use a bar and some weight as this - believe it or not - will actually help.