The goal of standing is to vertically stack your spine with a gentle S-curve from your tail bone up to the base your skull. If you look at your side profile, you should be able to draw a straight line down from your earlobe, through your shoulder, the mid line of your torso, your hip, knee and ankle.
This is my Pulp Fiction video/post (the video captures the movements you need to learn). I am showing you the ending first, so you know what you are working towards. You may find yourself coming back to this frequently and will progressively find it easier and easier to accomplish. To start with, standing the way I am describing may feel weird and be really hard, but will look a lot more normal than it feels. It will challenge everything you are bad at and your brain's perception of what is vertical. A mirror to look at your side profile can be an important tool to help you see what I am showing you, but you need to learn to progressively feel it instead of having to see it.
Step 1 is to learn to use your diaphragm to support your torso upward and stop leaning backwards. Imagine there are two half balloons inside the lower part of your ribcage that you can inflate by taking a deep breath into your lower back ribs. This should lengthen and stretch the lower part of your thoracic spine. It may also feel like it tips you forward. This is the part of your back that was accommodating your neck, but we are going to try and stop that. If you look in the mirror, you will likely see that it is your head that is forward and that your trunk is now actually quite vertical instead of tipped backward. See if you can keep that full feeling in the back part of your ribcage, but still be able to breathe freely.
torso leaning backwards diaphragm good spine posture
Step 2 is to learn to gently lengthen your neck up and resist the urge to pull anything back. In other words let your shoulders relax and hang and just lightly lift your head upward like I put you on a stretch rack. Don't forget about what you learned in step 1. At this point you may feel your shoulders are quite rounded forward, if so, don't worry about it. You are tight in your shoulder muscles and this will have to be dealt with afterward, for now let it be ok that your shoulders are a bit forward.
forward head posture lengthen neck up, not back
Step 3 is to get your pelvis underneath you and on top of your hips. You likely hang out with your pelvis in front of you and tipped backward or with your pelvis behind you and tipped forward. We will call the tipped backward people pink panther butt and the tipped forward people daffy duck butt. The pelvis confuses people because there is usually a need to adjust both the forward- back position of the pelvis to get it underneath your trunk and the angle of how the pelvis sits on top of the hips; they are similar but different things. The pelvis should be slightly tipped forward to most effectively balance on the hip joints.
Pink Panther Daffy Duck
If you are a pink panther, you need to draw your whole pelvis back underneath you and use your hip flexors and inner thighs to tip your pelvis forward. Imagine you are trying to pull your pubic bone down to the ground. It may create a light tension in your lower stomach and inner thighs and a spreading feeling of your sits bones in the back. If you have been a pink panther for a long time, standing straight like this will feel like you are really sticking your butt out, but you're not, you were just sticking your pelvis out before. Look at your side profile in the mirror, it should look straight if you have remembered the first two steps.
If you are a daffy duck, you are likely way tighter in your upper thighs than you realize and need to learn how to use your glutes and hamstring a lot more; they will push your pelvis forward and underneath you. Try to relax your knee caps and try to use the muscles in the backs of your legs to push your pelvis forward and underneath you. As your pelvis moves forward, make sure your knees stay straight; if they bend, you are just shifting your tightness to another joint. Again don't forget about steps 1 and 2.
Finally is step 4. Learn how to use your feet and how your shoes are affecting your standing and walking posture. Your foot should act as a tripod with weight distributed over the heel in the back and the balls of the first and fifth toes in the front. Your foot should function to lightly grab the ground when balancing and walking.
Details of each step can be found in future videos. It is best to start drawing your attention to one step at a time to help you create awareness of how you stand. You will find that trying to control everything at once will be overwhelming at first but as your body awareness develops, proper standing posture will become more and more subconscious. Have fun and good luck.