Spend plenty of time with natural breathing, you can make a ton of progress utilizing that method. It goes *very* deeply before it will sorta naturally roll over into reverse breathing - so really by the process of mastering natural breathing you come to understand reverse breathing more deeply. The point at which reverse breathing becomes the natural constant method of breath is really a fantastically accomplished level, so dont discount the profundity of a well accomplished natural breath strategy even though it may be technically lesser. The list is "good for academic understandings" but of course it is most effective to concentrate on one thing or one logical grouping of things at once before moving on or adding procedures.
As such, first tackle natural - we all know the general description, ribs relaxed; on inhale diaphragm descends, abdomen out, perineum relaxed, kidneys (quadratus lumborum) relaxed, sides relaxed; on exhale diaphragm relaxes, abdomen contracts, perineum lifts, quadratus lumborum gentle but firm not open, etc...
So for some finer points, consider
-the juxtaposition of the diaphragm to the psoas muscles - the bottom of the diaphragm overlaps with the top of the psoas;
therefore...
-at the very beginning of inhale, descend the psoas and use that little motion to begin the descent of the diaphragm
what this will do is "root" the motion of the diaphragm.
normally when people breathe they use the nostrils and sinuses to buffer the air pressures while the center of the diaphragm descends - but if the focus is to calm, this will not do since the center of the diaphragm are where the foramina are, so if the focus is about the center then there will need to be a tension there that assists the pressure-buffer of the sinuses to ensure a relatively smooth breath. through the various foramina pass through significant structures that we're also trying to calm, i.e. vena caval, aortic, esophageal (and the vagus nerve also passes through the esophageal.) now the problem arises that as one slows the breath and various bodily functions like the heartbeat, breathing in this unrooted fashion produces a hiccup in the diaphragm (not the standing wave "hiccups" but just a single one) due to the competing tensions between structures - and when you're balancing/buffering that actively with sinus pressure it works ok, but relax and relax and relax...and oops, there's this dissimilarity that emerges *because* the propagation of motion in the diaphragm isnt rooted somewhere...
so by beginning the breath at the bottom of the diaphragm, it allows you to create a wave-motion that propagates upward and forward as the diaphragm descends. that also blends motion in very nicely with the perineum, making it overall more harmonious.
as periodicities are slowed the notion of breath becomes much more focused upon "the central channel" simply by introducing this psoas-focus to the breath. once you integrate the psoas well, that is when you can truly "let go of the air passages" because doing it with a center-diaphragm focus they must be somewhat active to balance/buffer pressure otherwise the hiccup results (its back there...somewhere
)....
so, to let go of the air passageways I devised an active exercise to identify them so that they may be more effectively let go of - simply analyze every point air actually touches, from the tip of your nose to your sinuses to your throat, bronchi, lungs, and even diaphragm - "pull breath sharply" from every point you can find, one at a time. after a while from a position of movement you can better derive non-movement.
now with the air passages let go of, you can perform a very deep natural ab breath beginning with the psoas pulling down the bottom rear of the diaphragm and extending naturally to the perineum (and eventually, follow the kidney-sinew-channel right to the heels, K1 point.)
as you get things slower you notice that there are negative feedback mechanisms built in that...basically keep you alive, so you need to mitigate these as you get deeper.
-breathing too sharply you will find the olfactory nerves protrude forth into the sinuses and act as a sort of "mass air meter" just like in a car that will trigger the need to breathe, increased heart rate, as the rate of air flow over them increases
-an oxygen sensor that will trigger the need to breathe and it will also trigger a faster heart rate (blood concentration as opposed to air metering.)
-a tense diaphragm especially at the front will somewhat restrict the epigastrum and the heart will wind up tensed as a result, and when tense it will keep beating harder than you want - when proper the heart will feel light, buoyant, radiant
and any triggering of these mechanisms isnt a single transient spike but an induced oscillation, meaning when one of those triggers goes off it will take a corresponding amount of time to subdue the results. having mentioned the vagus nerve, it is a cranial nerve that runs to all of the zangfu and part of it going through the esophageal foramen it is part of what makes the epigastrum sensitive and it will relay the conditions below to above. as such, when everything is very harmonious, a corresponding harmonious-wave propagates back up the vagus nerve to the brain and it helps calm the rest of the cranial nerves - the CNs are where the senses derive from, so calming them is quite important.
once you get the hang of it, it wont take you too long to substantially and eventually, completely eliminate turbulence from your breath. it is truly a wondrous thing - turbulence is but many little vortices that form around corners and such, and all they do is extract energy from your breath. eliminating them you are able to extract as much as possible from the breath.
lots of this is for post-analysis and of course you'll make note of many things as you're meditating even though you're supposed to be keeping your mind quiet. but really, keeping your mind quiet is a matter of "training a path of least resistance" for the energy that you have nothing better to do with - haha - the concept of "thought-stream-energy" can be harnessed into
doing - so when one is doing, how can one be thinking, or whatever else, if one is truly immersed in the act? as such I've spent a lot of time merely trying to "execute the perfect breath" for my given cardiovascular rates and effecting the harmony in proper proportions, since trying to make too much progress too quickly one will run right into those negative feedback mechanisms I mentioned.
so really, plenty to work on before the list has even been considered