There are six basic requirements in doing good tai chi. The first three apply to the upper body. The last three apply to the lower body.
- Keep the head and spine straight (naturally straight, not arrow straight)
- Sink the shoulders and the elbows
- Empty the chest and expand the upper back
- Knees slightly bend and out (open your hips)
- Keep Yin and yang clear
- Keep feets at slight outside heel line
You can compare this to the Yang Style 10 important points by Yang Cheng-fu, referred often as one of the Tai Chi classics. You can see that these map to the classic 10 points, but in a more direct and organized way (but I am a little bias since I have these six points drilled into my head and body at this point).
I am always amazed how well these six requirements work. When one of my students have a problem with a particular movement, nine times out of ten it is because one of these requirements are violated. This is particularly true for the lower body requirements. Not following them results in wobbly stances, and awkward movements.