Make sure you 100% know exactly when the inside hind is lifting, pushing, and landing. Without that knowledge, you can’t really make a good leg yield. Lots of people go to leg yield and they don’t get the timing right and wonder why the horse is crooked and leading with his shoulder.
What are you doing with your seat? Are you sinking to one side, leaning in a certain direction, collapsing or twisting?
What are you doing to signal the leg yield?
I like to ask when the horse’s inside leg is on the ground. I ask by pressing my (inside) calf against the horse’s flank just as his inside hind is about to be brought forward, I maintain a consistent outside rein especially if he is the type to lead with shoulder, and I keep weight on my inside seatbone to push him outwards and over, one step at a time.
If the horse starts leading with the shoulder, don’t keep pushing with your inside leg. Correct with the outside rein and seat (and outside leg if needed), get the horse walking straight again, and try again. Don’t commit to a faulty movement.
Also make sure that when you ask, your horse is 100% straight. Get him walking on the quarterline and don’t ask until well after your turn onto the quarter line. if he is still bent around the turn, he’ll be crooked leg-yielding out.
It helps to have a groundsperson help you a few times first, and then see if you can duplicate it by yourself.