So, where better to begin than tadasana? Ray says there are 8 major steps in tadasana, but here are a few points that resonated for me.
1. After extending the spine (erector spinae), work with the balance of the pelvis. Squeeze the butt (activate glutes) and notice how that brings the top of the pelvis back and drops the tailbone down (and forward). The action of the glutes also externally rotates the thigh bones (femurs.) More subtly, gluteus minimus can lock the head of the femur into place.
2. All this back and butt squeezing pushes the low ribs forward. Activate the abs (rectus abdominus) to drop the ribs and activate psoas to draw the top of the pelvis forward. Find the balance in the pelvis between activating glutes and stabilizing pelvis with the deep hip muscles.
3. Activate quads, drawing kneecaps up. Activate inner thigh muscles (adductors) to draw thigh bones together. Counteract the glutes' externally rotating the thighs by scrubbing the feet apart while keeping the adductors fired.
4. Pull shoulder back - actually get an external rotation of the upper arm bone (humerus).
5. Activate trapezius to pull the shoulder blades down. Activate triceps to straighten elbows.
6. Squeeze the shoulder blades together (rhomboids) and hug them tightly to the back.
7. Now, with shoulder blades in place, try to roll the shoulders forward. You won't be able to - the shoulder blades are keeping the shoulders from moving forward but this action will lift the chest.
8. Finally, recruit those muscles on the upper side body (serratus anterior) to lift the chest. Imagine pushing your hands against the sides of a doorway to engage those muscles and lift the chest.
Wow, that's a lot for the "just standing" posture!