From a standing position: 1) Suck in your abdominal muscles, (thereby connecting the upper thigh to the torso). 2) When you can, draw the abdomen in further, allow the legs to bend slightly, but bear in mind that the bending of the legs merely allows you to suck the abdomen in even further, so
Note that, when you suck in your abdominal muscles, the pelvis rotates, causing your backside to tuck under, i.e. the tip of the spine will appear to tuck further between the legs.
Whichever leg you have the weight on during the step, make sure that the knee stays pointing at the toes; don’t let it collapse inwards (medially).
Make sure that the small of the back has relaxed and ‘opened’, which means that the ‘S’ bend has straightened out. (If you have sucked the abdominal muscles in enough, your bottom will have tucked right under, and the lower back will feel flat (no ‘S’).
If you have done the above, and engaged the core muscles correctly, you will feel stable.
James Drewe teaches in London & Kent. Details of classes can be found at http://www.taiji.co.uk.