imho, the rote method of getting these jhana stages should be the first method employed.
Instead of focusing on your nostrils, consider the fact that the olfactory bulb (one of the 12 Cranial Nerves) protrudes forth into the sinuses and acts like an air meter - the fast the air flow over it, the stronger the excitatory signal produced by the olfactory nerve.
Also consider that like an air hose, the larger the internal pressure of the hose, the stronger the walls of the hose need to be in order to sustain the pressure balance.
With that in mind, I devised an identification exercise for the air passageways - simply take some time and investigate everywhere the air touches and how you might use muscular input to affect the local air pressure there. So pulling air sharply at the nostrils activates the noseflaps, try the crown of the nose, etc, all the way down (but do go gently on the sphenoid sinus, too forceful there and that's simply a recipe for a headache) - then from a position of action, derive inaction: let the sinuses and air passageways go completely.
This turns out to be very relevant the more you slow your breath down. Eventually you want the preponderance of motion to be contained within the Diaphragm-Psoas-Perineum dynamic. One thing that hinders progress in that area is the buffering of air pressure provided by the sinuses - basically by using them to facilitate the movement of air, you are taking emphasis off the diaphragm.
Make sure you begin the descending motion of the diaphragm from the lower rear (inferior posterior) and continue that motion down, down, down, for the entire length of breath. What this does is "root" or "anchor" the breath - the diaphragm connects to the anterior longitudinal ligament near the lumbothoracic junction, adjacent to the psoas muscle - so you can just about include a very subtle waxing and waning of the entire spine when breathing deeply. Part of the reason for anchoring the diaphragm motion is that you want to let a smooth wavelike motion pass over the foramina in the diaphragm - the very sensitive openings through which pass the Vena Cava, Aorta, Esophagus, vagus nerve. The smooth motion also helps avoid stimulating the vagus nerve, which will also hinder progress - rough motion there will invariable stimulate the heart and lungs to do their thing.
So basically getting to the very deep stages requires this streamlining of the breath mechanics, consciously or not. The idea with this rote method is to bring the flow of air beneath the threshold of turbulence in the air passageways - turbulence is basically little circular vortices that for around the nooks and crannies that steal energy from your breath. Below the threshold of turbulence we dont get the Cranial Nerve stimulation that sets off neural firing chains that eventually result in higher brain stiimulation - aka the manifestation of thought-stream-energy.
Bringing the focus to the rote physical in this way blazes a secondary path of least resistance for this root-core energy that you may harness via the practices before it manifests as thoughtstreamenergy, leading to a calmer, clearer mind.
Of course one may get there with other methods, but this is what I found worked best for myself over the last ~8 years of my studies