How to breathe?

you have to switch to sensations

Anapanasati is broken into 4 tetrads out of which the 2nd tetrad is Contemplation of the Feeling. Once you are in step 4 and moving to step 5 you are moving from Contemplation of the Body to Contemplation of the Feeling.

When your breathing stops next is to keep experiencing the itching, perspiration, heat / cold, irritation feeling at the centre of the upper lips continuously. At 1st these are the types of sensations you feel but after sometime it becomes pleasant. This is when you really are in step 5 and before that is more a transitionary phase. If there is any tension you have to release it by keeping pasing you attention though these areas.

I downloaded the manual about Anapanasati which that answer recommended, and I hope I get a chance to read it, but it might be useful to read some other shorter/different answer of yours here.

You should already have a teacher to hold hands and guide you or you should do your homework properly. Read through many relevant Suttas as possible. I have given many references below but also following maybe a good initial read: Samma,ditthi Sutta. It is dangerous to do without guidance or doing your homework properly as Breath Meditation done wrongly can have lead to complication.

how to breathe when meditating

You do not have to breath in any particular way. You take a step aside and look at the process of breathing. Now the breath starts, now it accelerates, nor it has peaked, now it is slowing, not it has stopped, now there is a gap, now it has started in the opposite direction, .... Then at end of each in breath and out breath where you with the breath or did you mind wander away. If so this it create any tension, relax them by passing you attention though these areas in a zigzag way if large of is small, keeping your attention for a small time on the area. Then get you attention back to the process smilingly without any agitation or worry. Continue doing this.

When you start choose the rest your attention in the triangle from the top of your nose and base of the upper lip. As you become skillful narrow it down from the tip of the nose to the base of the upper lip. Then just the upper lip and finally the centre of the upper lip.

Also 1st start with the process of breathing then followed by, touch sensation of the breath, and then finally keep the breath on a spot at the centre of the upper lip.

My breathing gets shallow and then stops

Keep watching you bodily sensations, the sensations will initially be unpleasant, than pleasant and finally neutral.

Q1) Initially you breathing will be deep but as you go on it will become shorter and stop. When it stops it can be a few minutes or even the rest of the whole session. When this happens at 1st you become restless and sometimes wonder what to do next. When you worry and think this actually means you need oxygen. The natural response of the body is that you start breathing again. Clearly this is not happening or you have intentionally stopped breaking or holding you breath even perhaps for psychological reasons because you want it to stop. You should not in any way manipulate the breath, though when calming the breath you can indirectly doing by increasing your focus and actively reviewing if the mind is with the breath and actively redirecting your mind to the breath. When driving generally is is better to concentrate on expansion and contation of the body than focus on the tip of your nose as it is less prone to create drowsiness. If the activity is not hazardous (there will be no accident if there is a lapse) continue with the sensation around the mouth or if possible the centre of the upper lip also being mindful of the breathing process.

Q2) This is coming from Yoga and Buddhist sects influenced by it so I am not an expert. But having said that for it the be insight meditation you focus should be in the 4 elements (hardness, expansion contraction, etc) or the feeling you have around this area.

Q3) This again is more like Pranayama though you are not intentionally doing it. This creates some nice feelings but again this is not insight meditation. Sun Lun Sayadaw teachers something like this but when doing this without guidance is not advisable. Also the method of Thanissaro Bhikkhu and Ajahn Lee does take a more active approach in manipulating the breath.

Q4) Anapanasati: Mindfulness with Breathing - Unveiling the Secrets of Life, Ānâpāna,sati Sutta, Ānâpāna,sati Sutta, Anapana Sati Meditation on Breathing by Ven. Mahathera Nauyane Ariyadhamma, Mindfulness of Breathing & Four Elements Meditation by Ven. Pa Auk Sayadaw, Knowing and Seeing by Ven. Pa Auk Sayadaw, Mindful of the Body: A Study Guide, Right Mindfulness: Memory & Ardency on the Buddhist Path, Keeping the Breath in Mind & Lessons in Samadhi, Inner Strength & Parting Gifts, With Each & Every Breath: A Guide to Meditation, though not strictly related the following answer also has many references which are relavent to making your breath meditaion sucessful also: https://buddhism.stackexchange.com/a/13210/295, I have quite a few other answers in this are also which I am having a hard time re locating.

Q5) If you do not enjoy breathing normally maybe you are building up tensions. You should do this releasing body and mind by being aware of tensions in the body and mind and passing your attention to them. Also you might be having Restless Worry when doing breath meditation. When you are doing other activities perhaps you can do contemplation of the Wind Element (expansion and contraction)

Also with your medical problem I am not sure if Breath meditation is the best thing, but unless you try you will not know for sure.

Apnea is a serious medical condition. I had an otherwise healthy colleague who had to be plugged to a machine overnight, to monitor his breathing cycles and supply oxygen when needed. This is no joke and requires professional medical attention.

That said, some of it may come from your deeper emotional blockages. What I would try if I were you is tapping of abdomen, diaphragm, and chest. Just tap one of those three spots, for 5-10 minutes at a time, with your two fists like King-Kong, or with your fingers if you are very sensitive. Chances are, you will discover layers and layers of blockages. Try to relax muscles of the area you tap. The more you let go, the less painful it becomes, but then you get to the next layer and it may get more painful again etc. Just try it gently. Ask your doctor before doing this, just in case.

Tai-chi and singing are good too. I encourage you to do more of those :)

As for your experience during meditation, there is no hard boundary between "watching" and "controlling". To the extent that you are watching your breathing, you are already interfering. The periods of no-thought between the breaths are fine, but they should not be associated with dullness. Breathing and discursive mind are related, so your inner monologue may pause or stop, but the rest of your mind should go on fresh and vibrant. When things go well you will fall into access consciousness, when you get to "see" directly what normally is your subconscious. It is a rather peculiar experience, akin to a drug trip.

I don't know to what extent you are similar to me, so not all of my experience may even apply, but in your meditation try not to worry about breathing so much and focus on the feelings in and around your body instead, same areas I suggested you tap can be used as doors into the subconscious.

Well, it's one of two things. It could be related to your apnea. Like Mr. Silva says, I also am no doctor, so I can't comment on that.

However, when you wrote this - "I'm no longer aware of what I'm seeing and/or of what I'm thinking", you got me thinking. If you didn't have sleep apnea, and you wrote that line, I'd be shaking your hand and saying congratulations - you've stumbled upon a major milestone in your meditation. That is the opening of no mind - what we Zen folk call mushin. Obi wan would have called this your "first step in to a larger world." That blankness is where all the fun happens. It's the boundary between yourself and your bigger mind. It's ultimately what you are aiming for.

When I'm awake this period of 'unconsciousness' is probably (I guess) no more than half a second (though the period between breaths may be several seconds), and only happens while I'm physically at rest (not physically exercising) and emotionally dull (I think I keep a relatively lively interest when I'm driving a car for example).

For most people during waking consciousness, this period of no-mind is only a second or two. As your meditation develops, the period begins to last a bit longer. Eventually, this will grow into minutes. As you say, it may appear as dullness at first. A common misconception people have is that meditative concentration is always marked by lucid clarity and the kind of alertness you might have when reading an engrossing book or playing and absorbing game. It's not. Don't let that get you discouraged. Meditation is a totally different way of using your mind. There is an adaption period that you are going to have to go through.

To tie this back to breathing... I think where a lot of people run into problems is when they try to breath naturally. "Try" and "naturally" are antagonistic. A better way to go about it is to simply watch your breathing. Don't worry about the length of the breath or even [for now] what part of your body is rising or falling. Follow the breath all the way out. I mean allll the way out. Don't short change it! A habit that a lot of people develop is that they drop their attention about 3/4 into the exhalation. Next, keep that attention applied during that blank space between breaths. There is a space there and you need to keep your attention on it. Lastly, don't let that concentration break as the air starts to come back in. Another habit that people have is that they let their concentration go lax once the outgoing breath and blankness switch back to rising. Don't do that. Keep you attention fixed.

It sometimes helps if you anchor that watch point. Some people use their nostrils, others might pay attention to the belly, personally I use the tip of my nose. That's mostly a matter of personal preference. The ideal here is unbroken, fixed attention.

Frankly, it sounds like you are doing better than you're giving yourself credit for. Don't worry about breathing right; just let the breath be!

My oxygenation depletes, which seems to manifest as a slight loss of consciousness

That does not seem common/normal -- I'm not a doctor though.

Maybe it helps if, instead of focusing on breathing instructions (which might be problematic in this case), get enough information about what exactly is to be accomplished (and work your way towards it).

Restricting ourselves to the first stage of anapanasati (samatha), breath should get calmer and calmer. However:

• this is done in concert with your entire body and mind.

  Breathing patterns and oxygenation changes the behavior of the physical body/brain and mind. As I understand, they get calmer, and allow the breath to get even more calm and so on.

• You should not lose consciousness.

  Perhaps you might need more conscious intervention in the breath to avoid this.

• Your concentration should get stronger

  That's a main goal. Your thoughts should get very very calm, your alertness should get very strong, along with the ability to concentrate on a single point for long periods of time without being distracted (even slightly).

Am I interpreting "breathe naturally" a bit too literally [?]

I believe we are taught to "breath naturally" to avoid any creativeness and excess on our part while breathing. In reality, we do need to intervene in the breathing process to make it calmer. Otherwise, it will just be our "everyday breathing" and it won't get calm. But we can't be excessive -- in fact, very little force is usually too much force already. One does not force it into tranquility, but one does apply some control, mostly, avoiding gross in-out breaths and relaxing "micro tensions" wherever one identifies them.

Some people teach students to make a resolution to be calm before meditation (by expecting the mind to "take the inclination hint" and do the calming without conscious intervention). Others teach something like zazen, or an open ended observation of sensations, so one gets familiar with the subtlest things that makes the body and mind a bit calmer or a bit upset. Once one understands how to observe, one can actively drive the breathing, body and mind to a calm state.

The 'Anapanasati manual' which I reference above barely mentions "diaphragm" nor even "abdomen".

Samatha is usually taught on a fixed point -- I'm not sure it would work with the focus moving. A favorite point is the tip of the nose or right below it. I suppose one could try focusing on a fixed point in the diaphragm region, but it might be challenging to localize and stick to that fixed spot.

However, mindfulness/satipathana have been taught using the diaphragm moviment as meditation object.

Is there some written text about 'how to breathe' that you recommend?

Here's a short pdf on breathing. This is what I read that made me look for authoritative resources (as I realized I know nothing about breathing). I ended up buying Breath Well, Be Well by Robert Fried, Ph.D, which I'm still reading.