I start with the base of the base, meaning what my body is actually resting on - my feet or knees on the floor; buttocks on chair, stool or cushion. I feel my perineum (area between genitals and anus), give it a squeeze (like trying to stop peeing and stop a fart at the same time). I feel my out-breath go down to the perineum area and relax it; this is the area of embarrassment, anus and genitals, that most of us have locked up. I feel my tail (base of backbone - coccyx and sacrum) relax and soften. We often relate to the sense of 'I' as being in our upper body somewhere, so here I redress this balance a little, giving attention to my lower body.
Having got the base of the base in mind, I move my attention up to the lower trunk; I feel my hips and perineum like a cradle, supporting the rest of the body. I breathe out down into my lower body - belly and/or perineum. At the end of my out breath, I have a range of possible squeezes, from energetic (perineum contracted up into my body, belly sucked in towards back bone, as if someone with very hot hands is about to touch my belly), through a definite perineum squeeze, to a slight perineum squeeze, to no squeeze at all, just directed out-breath into the perineum-sacrum area. How much energy I put into this squeeze, if any, is a function of how I feel. If I am sleepy, or mind wandering, can't concentrate, then I might put a bit of energy into the squeze. If I am relaxed and mellow, probably no squeeze at all, just a mental following of the breath out and down into that area.
I generally follow, or even direct, my out-breath down the body to the base, into my pelvis basin; I then relax, and let the in-breath take care of itself. It is like having a sponge in the belly that the out-breath squeezes, and when I relax it expands automatically, causing an involuntary in-breath into my thoracic area - ribs, chest and upper body without effort - belly slightly extending. Of course, I realise the actual air in my lungs stays in my lungs, and on the out-breath the air goes up and out of my nose or mouth; it is not going down into my lower body. So when I say I follow my out-breath down and out to my perineum or belly area, I am talking about what I experience as the energy of my breath. In actual fact, I believe one of the main secrets of body-based breath meditation is to allow yourself to imagine that you can breath in and out from anywhere in the body, and that the air is not limited to the lungs, but can fill the whole body (even though doctors tell me this is not what is physically happening). The other secret I believe is to put directed energy into the out-breath if you like, but to always allow the in-breath to take over without interference.
After my directed out-breath, as I relax and let the in-breath take over, my belly will usually expand; this is often called 'abdominal breathing', but the body is three-dimensional. As my belly expands frontwards and outwards at the beginning of my in-breath, I am also aware of side expansion, and my back as well - maybe my lower back does not expand backwards, but I at least allow it to soften, and not tense or contract inwards (as often happens when people concentrate on abdominal breathing).
I move on up my trunk to the chest area, with the same three-dimensional observation - as my chest expands with my in-breath, I also feel my back, the shoulder blades and whole shoulder girdle soften, and I allow my shoulder blades to separate on this in-breath. My chest is soft at all times (my base is soft on the in-breath, only tense and energetic ever towards end of my out-breath, but not always even then). I feel my chest and shoulder-blades cross-section aligned on top of my belly and lower back cross-section; and my belly and lower-back aligned on top of my base. I move a small amount very slowly; after each tiny move I wait for a breath or two before deciding whether I am more aligned (feels better) or not; I don't move and then decide immediately if it is better or worse. This is like a very slow motion, imperceptible 'wriggle'.
I also feel expansion of the in-breath in the upper chest and shoulder area. Sometime I imagine I am a giant sponge (a different sponge from my previous example!), and I am breathing in from all over my body at once. My upper body is softening and expanding with no effort. I breath in from anywhere and everywhere, I breath out down and into belly and base.
My shoulders are often tense, and it is difficult to soften them; they don't seem to respond to my suggestion to relax or soften (unlike other body parts, which respond readily to my conscious wish to soften). One way of relaxing them is to make sure my hands are resting near my body trunk (not away from my body, such as on my knees) and to consciously let my arms go heavy, and breathe into them. Feeling the warmth and energy in my hands helps, particularly in the palms.
I align my head and neck on top of my trunk. Most people need to put their chin down a little; I don't find it easy to soften my head and neck area, and get comfortably aligned, but one part which is easy to consciously relax is the lower jaw. There are also several images that can help alignment of the head: the head falls forwards and up (Alexander technique); the head is suspended by a thread from above, attached to the crown; the head is like a ball balancing stationary on a fountain. I find it helpful just to be aware that the joint my head is balanced on (the top vertebra, the 'atlas') is located approximately between my ears - I used to think of my head as being balanced far away from there, in my neck somewhere. I move my head, neck and shoulders very slowly to find alignment - again, pausing for a breath or two after each micro movement to ascertain if it is better or worse.
I soften my neck, throat, and face - chin, jaws, forehead; let my eyes float free in their sockets. I also breathe into my head; like with the chest, I sometimes breathe in direct through all areas of head (breathe in from wherever I like, breathe out down and through base).
Throughout the sitting, I am adjusting my alignment, my relaxation, my resilience.