Use a pool if possible. Practice in very shallow conditions, so 1 to 3m, as there pressure changes are large by small depth variations.
Some things are:
Try to change and maintain depth purely by breath, do not touch the ground. Use somw fix point, some tile or so.
Try to do this without moving.
Repeat while doing something else, for instance take off mask or switch to backup regulator or use a cam to take pictures.
Put some weights in the bottom. First try to stay at bottom with max air in bcd. Then pick up weight until you cannot stay buyont any more, do not change the air in your bcd.
Repeat the same but take more weight than in the previous step and then try to use as low air in the bcd as possible. Let go of weight piece by piece and try to stay floating.
Then very slowly fin around at max depth 1m and maintain depth. Repeat without mask (some distraction).
With all try to get a horizontal position. Let someone observe and maybe film you. This person should push you in the right position. Try to feel comfortable by moving your leg position, belly position and muscle tension a bit. So try to feel natural. Maybe try to be a little head down to balance air in the bcd or if it does not work adjust your tank position/ weight position and try again.
Finally.make a parcour like with rings, so you have to change depth swim through, chsnge again turn and so on.
Do everything without using your bcd.