Ok, so here is an interesting method to establish the MCO by using the Bigua waxing and waning set of hexagrams:
I had to revise this to give a bit more of a conceptual introduction to the practice...
Traditionally, the microcosmic orbit has been taught and practiced by drawing qi up the governing vessel in the back and leading it down the conception vessel in the front. This method aims to abandon the idea of leading the qi and instead, build from the very basic fundamentals of breathing practices, and directly work with the 3 dantiens energy centers which will serve as the catalyst for the practice.
As the fundamentals of breathwork are performed, the signals of the body's breathing mechanics are streamlined and attenuated to a very significant degree - resulting in utter and complete stillness; the very essence of anapanasati. This is a very important practice point that must have a certain level of mastery attained by the practitioner if any strong energy moving practices are to be done. The energy is best integrated in utter and complete stillness, it is every bit a fundamental point as it is a safety mechanism for the energies raised.
After the fundamentals of breathwork are accomplished, embryonic breathing and then lower dantien breathing are utilized to develop the energy center of the lower dantien. I differentiate the two as such: embryonic breathing is works with the gut to develop more sensitivity so that the fundamentals of anapana may combine with the somewhat concentric circled nature of embryonic breathing, with the lower dantien point at the center. After proprioception is developed and the lower dantien begins to reveal itself more significantly, then the approach can be used to start working with the lower dantien itself - therefore when I consider lower dantien breathing, it is using one's intention to harmonize and work with the waxing and waning of the energy center itself along with the breath. Heat and light are two common manifestations of some success with working with the lower dantien.
Next in the process is locating the upper dantien, the niwan, at the pineal gland in the midbrain. By bringing the focus of awareness in to the niwan, a measure of energy is prevented from manifesting itself in the cranial nerves, how our senses extend themselves into the world. Dr Yang, Jwing Ming described the process as "condensing the shen at the upper dantien" and Luk's translation of Taoist Yoga described it has "fixing the spirit at the seat of awareness"...as if one is simply placing a lamp in an empty room. This technique must be done gently, there is simply no muscular action that may assist this process. Another good description touted by Drew Hempel is "finding the source of the I-thought" as this identification with "I" resonates at the seat of awareness. This technique may be employed to great effect as an added technique to anapanasati; when combined with lower dantien breathing the results are multiplicative and an eventual harmonizing of the upper and lower dantien takes place. Eventually one may wax and wane the two energy centers in unison to increase the process of harmonizing the two.
The third component is the middle dantien - the solar plexus, translating to the celiac plexus from which all of the lower zang organs - it innervates the stomach, spleen, kidneys, duodenum, liver, just to name some of the most significant innervations. One good and potent technique I used to develop this was via Yan Xin's 9 step method - after having established the fire at the lower dantien, an idea of a lotus flower at the solar plexus is used to further develop that energy center. Eventually, a similar waxing and waning of the energy center is accomplished, and harmonization takes place.
These 3 pieces comprise what fits into the larger framework of the microcosmic orbit. When we begin applying these 3 to the concept of the chinese Hexagrams, it emerges that the yin or yang lines wind up relating to what the energy centers are doing. The yin or yang line is a direct expression of the energy center in question. The method in this instance utilizes reverse breathing, and considers the hexagrams split into two trigrams - the lower (inner) being the inhale, the upper (outer,) the exhale. Relating to the beginning stages of Taoist Alchemy, the reasoning for the middle dantien being the solar/celiac plexus - for further reference see the nature of the cauldrons in works such as Taoist Yoga or Foundations of Internal Alchemy - so the middle dantien only equates to the heart center after the generative force and vitality have been fully restored and the 'mating of dragon and tiger' happens in earnest and the nature of the cauldron-configuration changes.
To begin relating the concepts directly to the hexagrams, let us consider lower dantien breathing, hexagram 24:
There is a yang expression about the lower dantien as the reverse abdominal breath inhale is executed and then a relaxing yin expression on the exhale. Personally, I feel there should not be too much emphasis on constricting the front of the abdomen in learning reverse breathing since it places an unnecessary focus on the front. The fundamental breath mechanisms, from an internal energetic standpoint, are the diaphragm, psoas, and perineum - I came to this conclusion from deep longevity breathing as a result of my own practice - at some deep point, "the breath externally disappears" and this is the result of significantly harmonizing the breath mechanisms and calming the body to a very low consumption state where every process must become as streamlined as possible. Eventually one is able to breathe without moving the front of the abdomen - combine that with Dr Yang's notation that the 'false dantien' qihai point lies along the MCO route, thus qi generated there tends to into the loop and utilized.
Once such a calm, deep state has been reached, that bottom line is really no longer yang either - so the deep utter and complete stillness resultant of longevity breathing can be correlated to hexagram 2 - a perfect way to end most any energy practice:
This covers the most basic conceptual and safety mechanisms for the practice.