Part 1 - Postures
When you walk you must be aware of all the movements of the foot, slowing down your stepping. When you sit you should note the upright posture of the sitting. Not the form of the body, but the upright position of your body must be noted as sitting. When you focus your mind on your sitting you know that you are sitting. Then you note sitting.
In Burma some meditators when they are instructed to note the sitting posture find out the form of the body, the shoulder, the leg, the eyes, the nose, the head. Because they are looking for the form of the body they couldnt note it. But the Buddha doesnt instruct us to note these forms of the body. What the Buddha instructs is to note the upright posture of the body as sitting, because he would like us to realise the supporting nature of vayo dhatu, the wind element. When you sit theres an air inside the body, and also air outside the body. The two airs support the body so it is sitting in an upright position. So to realise the nature of the supporting wind element the Buddha teaches us to note sitting. So you should focus your mind on the upright posture of the body and note it as sitting. In the same way the upright position of standing must be noted: standing standing standing, sitting sitting sitting and so on. Sometimes some yogis misunderstand this instruction so when they are instructed to note the sitting posture what they notice is the contact between the body and the floor or the seat. Its wrong. Thats contact or touching, not sitting.
The commentary to the text explains that sitting means the bending posture of the lower and the upright posture of the upper body. I instructed you to be aware of the upright posture of sitting, the upper body, because if you go down and be aware of the bending posture of the lower body your mind tends to go to the contact.