Imagine your foot exists of three points: Your heel, the ball of your foot, under the big toe and the ball of your foot but then under the small toe.
Let's get started: Make sure you do this exercise on bare feet or on socks. Now stand on the floor with your feet hips wide. Now bend your knees a bit. First try to divide your weight evenly over the three points of your feet, for both feet. Move your weight gently from any combination of two points to a another combination of two points to get a dynamic feel for it. And now comes the magic part. Move your hips a bit forward! Awesome, you are doing it! Now while your are moving your weight around on those three points you are using your hips to keep balance.
The problem with walking on shoes with heels is that you involuntarily start using your head to keep your balance. Once your are standing bare foot and train yourself to use your hips again, what should you do with your head? Relax it and let it follow the lead by your hips. Become aware of your breastbone. Imagine a stick is being connected to your breastbone. Now hold that stick and move it up until you get the feeling your are looking 45% up and you curve your spine backwards. The next thing you'll notice is that the tension in your back disappeared. The fact is you're not looking backwards, you're looking straight, you're not curved backwards, you are standing up straight!
Standing up straight is not a tense, stiff position, it's completely dynamic and relaxed. It's more like dancing, althought the movement can be so subtle that nobody sees you moving. So this whole process is not just an exercise, it is standing up straight!
Probably you have been using the same two points for both your left and right foot, that's the basic exercise indeed. The next exercise is to use different points.
If you have a partner you can ask your partner to move your hips around randomly. All you have to do is follow the lead.