When you bend over to address the ball, you must not lower your hips at the same time. Keep them where they are to create a level swing and maintain your balance.
Try this. Stand upright and put a 5-iron on the ground right against your leg. It doesn’t matter which one. Notice where the top of the grip hits your hip. Now lower yourself into your stance like you usually do. If that spot on your hip is now lower than the top of the grip, you’ve made the mistake I’m warning you against.
Try again, this time taking your stance while keeping your hip where it is. It might help to think that you raise your hip up and back as you bend over. Actually, it will stay in the same place. Don’t forget to bend your knees if you have to.
Do this over and over until you get used to it, then take that 5-iron into your hands and swing it. I believe you will find your swing to be a lot smoother than it was before, and you will finish with very good balance.
This is one of those little things that you don’t read about in the instruction books. All the good golfers do it, though, just watch on TV. You can do it, too.