Source: Katy. Bowman "Rethink your Position"
1) In minimal or fitted clothing, stand sideways in front of a full-length mirror in the way you might stand at a checkout. Check out the position of your head, ribcage, and pelvis.
2) Find the front of your pelvis or the pokey hip bones. Do they line up with each other? Is the head in front of the chest or in front of the hips? Is the chest/ ribcage lifted, or is it out in front of the pelvis? Is the pelvis out in front of both the head and chest?
Once you've assessed your 'before' alignment,
1) Shift your hips back so your hip joints sit directly above your knees and ankle joints in a vertical line.
2) Adjust your pelvis. Your pelvis can tilt forward and backward, but a neutral spine is created when the pelvis is neutral (in between forward and backward tilt)
To find neutral, first locate the top, bony protrusions of the pelvis, called the 'anterior superior iliac spines' (ASIS), and the bottom, front point of the pelvis, called the pubic symphysis (PS). Looking at the side view, stack the ASIS directly over the PS.
3) Adjust your ribcage. Your ribcage is somewhat cylindrical. Often, when we 'stand up straight', we tip our shoulders back and move the bottom of the ribcage forward. Unfortunately this excessively increases the compression in the lower back, which is not great for the bones and discs in this area.
If your ribcage is tipping back, then tip it forward to align the front of the ribcage so it is stacked over the front of your pelvis. Be aware, if you are used to tipping the ribcage back, tipping it forward until it's straight will feel like you're slouching (the next step will help reduce that feeling)
4) Adjust your head. Spending lots of time looking down can mess with our spines. Whether we're looking at a screen, book or phone, our 'dropped head' activities lower the chin toward the chest (which flexes the vertebrae in the neck) and move the head out in front of the body (which flexes the vertebrae in the upper back)at the same time. The 'tech neck' creates a flatter spine in the neck area and excessive rounding of the upper back.
To reset both the upper- and middle- spine curves to neutral, reach the top of your head toward the ceiling while also sliding your head back (don't lift the chin) as you bring your ears back towards the shoulders - all the while keeping your ribcage in neutral.
Posture Focus.
How to find a neutral spine.