Under normal circumstances you should be able to breathe just fine when the back of your tongue is up on the soft palate. The people who say they can't breathe when the tongue is up there have recessed maxillas and/or very narrow palates, I think. 

I've tried to move my tongue into a position where it stopped me breathing and I can't - even when the very, very back of it is pushed up to the point where my gag reflex is yelling at me to stop, I can still breathe through my nose just fine. You are talking about breathing through your nose, right? Because every now and then I get this strange idea that people are trying to breathe through their mouths at the same time as mewing, lol. 

It does feel uncomfortable to engage the soft palate at first, especially where the uvula is located. Many people feel a gagging sensation or the feeling that they're on the brink of choking. This is just the body reacting to something it's not used to. After a while, the gagging feeling stops. People also report a "sore throat" feeling, as if the palate just isn't used to being touched like this. This subsides as well. 

If you feel this slightly uncomfortable gagging feeling then you are properly engaging the back of your tongue against the palate. Stick with it. If you can breathe fine when your tongue is up all the way back there, you're doing well. 

Getting the back of the tongue up there with a suction hold is one thing - making it stay up there is quite another. It's going to be difficult until your tongue develops good strength. At first you'll find it falls back down and you're constantly putting it back up. The act of doing so over and over again will help your tongue get stronger, so keep at it. You'll find that it sometimes "tears free" of its suction hold, and you hear a little sound of air movement when it comes free. Just put it back up. 

Basically what happened to me was that it felt like the suction hold was the only thing keeping it up at first, and that as soon as the air seal was compromised, it would fall back down. And I didn't like the feeling that I was just sucking the soft palate down to meet my tongue (at least that's what it felt like). Once my tongue got stronger, however, and I gained more muscular control over the back of it, I was then able to exert a comfortable amount of upward pressure to keep the tongue up there with muscular power and not just the suction hold. To be honest I think the proper resting position is a combination of both - suction hold and muscular force. 

What you have to remember is that your tongue is a very powerful, dextrous muscle and that you don't yet have full control of it. At first it seems like you're never going to have enough control to get the tongue flat against the whole palate. But after weeks of practice, you find it doing things that it wouldn't do before. You find yourself having conscious control of the back of it, whereas before you weren't even aware that you had control of the back of it. And it gets so much stronger. You can speed up the strengthening process with tongue chewing (see Mew's video on that). 

Maybe you aren't as recessed as you think. But to be sure I would be doing more than one "exercise" to find the right position.

Some suggestions would be saying "kkkk" in order to move the hyoid bone (the tongue's root) like a bird (?) would and raising it's awareness, so that you can be sure  tension isn't dragging it down when you try to mew. 

Some people find it useful to block airflow with the back of the tongue to be able to feel this area as well.

Be sure to use your sense of touch as feedback, to be able to discern whether the soft palate (including uvula) are touching the back of the tongue or not.

Try also to do nasal breathing exercises, like the Nadi Shodhana/alternating nostril breathing, to check if there is vacuum when you mew. If you feel your diaphragm and tongue still raise when doing it, it means the tongue position you had wasn't enough. But if you feel you already had the vacuum, then you were in the right direction.

And very very important is to master the cheesy swallow, with water and/or saliva. Then to master the suction hold exercise. That's what helps creating this vacuum that raises the whole respiratory system (I can feel how my diaphragm raises when doing it, since it's also connected to the hyoid bone).

And I would also keep using the nnng exercise to help bring the back tongue in contact with the soft palate. If it doesn't happen, it could be there is some tension on the area, that you are not aware of.

Here is some wikipedia info on the velar consonants ( https://en.wikipedia.org/wiki/Velar_consonant) and specifically the Velar nasal consonant ( https://en.wikipedia.org/wiki/Velar_nasal).  

In the first one, it states that "normal velar consonants are dorso-velar: The dorsum (body) of the tongue rises to contact the velum (soft palate) of the roof of the mouth."

In the second link, it states that the velar nasal "nnng" is occlusive, meaning it restricts airflow to the throat, and in this case shifts it to the nasal cavity.

Or I found it in this webpage as well, http://voiceguy.ca/blog/voiceguy/soft-palate-floating:

"Turn your attention to your soft palate. If you can't immediately feel it, take a second to make the shape of the "ng" sound at the end of a word like "song" (IPA [ŋ]). The back of your tongue should touch the soft palate. "

Edit: 

I agree with AlphaMinus on the muscle strength and Tongue chewing. Sometimes I do a nnnng and turn it into a Tongue chew, to feel that I am doing it with the back portion of the tongue as well. But it's possible to do that with just your awareness and learning which muscles are involved.

And also, Mike states that the suction hold is the mixture of a push and a suction. So muscle force should be already part of the suction hold.

OP, I forgot to say, be sure you are balancing your skull at the Atlanto-occipital joint, on your spine, to be really able to raise your tongue from it's root.

Edit 2:

And not only the sense of touch as feedback, but also the sense of taste and Proprioception/Kinesthesia.

You are talking about breathing through your nose, right? 

OP, I forgot to say, be sure you are balancing your skull at the Atlanto-occipital joint, on your spine, to be really able to raise your tongue from it's root.

So can I still mew if I'm laying down on bed or reading for example while I'm sitting, or is it only possible (at the beginning) to mew while I'm standing or sitting properly, straight, on a chair? 

Posted by: Antares

OP, I forgot to say, be sure you are balancing your skull at the Atlanto-occipital joint, on your spine, to be really able to raise your tongue from it's root.

So can I still mew if I'm laying down on bed or reading for example while I'm sitting, or is it only possible (at the beginning) to mew while I'm standing or sitting properly, straight, on a chair? 

You can still mew while resting and standing or in any position, it's just that when we are not resting, there can be a lot of tension from other muscles (specially in the neck area, which disorganises everything, dragging or misplacing bones here and there and not letting your body use a position of mechanical advantage in relation to gravity).

This tension can come from stress or because learned bad habits in childhood that were never addressed. It's okay to have a moment of tension when stressed or surprised, but it's important to learn to release it. You can do it with body awareness and attention, to learn to feel this tensions, and a lot of practice and intention to be able to free them. Even your tongue could be tensed, and that is also a setback in mewing.

It also helps, to let your skeleton system to do it's job and take the weight and move your body at it's joints, while the muscle organise comfortably around it. It sounds obvious, but it's not. People have a lot of misconceptions about their body..like having two jaws instead of one, or thinking the tongue it's smaller than it really is. And your brain believes it...it will move according to how you think you are built or function, so the more misconceptions you have, the more tension there will be. (But I'm digressing now)

I also remember reading that jamo/neveragain said he learned to be more aware of his "body mechanics", so I'm sure it's important in the mewing journey (at least it's been very important to me).

If you already are balanced and move with poise and grace, then you have a good foundation to mew. (It's more likely, that people who are really balanced are already mewing without knowing).

I just noticed all that because a few minutes ago I put myself in front of a mirror and tried to master the movement needed to put the back of the tongue in contact with the soft palate - a thing that this morning I couldn't do - and with my surprise I managed to do it! I just listened to Ayla's advice (the "k" thing).

The only problem now is that I struggle a lot to maintain that position and I cannot really breathe properly because the tongue is blocking the airflow. But I guess it's normal, since I just learnt how to do it. 

So, the next steps are to understand how to maintain the tongue in contact with all the palate and then to apply the suction hold while pushing with the muscle force, right?

Do you have any more advice? 

Posted by: Antares

So, the next steps are to understand how to maintain the tongue in contact with all the palate and then to apply the suction hold while pushing with the muscle force, right?

Do you have any more advice? 

Yes, but it's up to you which order helps you the most. And it may vary in your journey. In other words, do whatever you can to raise your tongue up there. You can even play with it, sometimes starting with a suction hold and ending with a nnng and maybe then doing a cheesy grin to see how it feels, or the other way around, starting with a nnng and then  suction hold. You are the boss, you decide what works best for you and the exercise/s to which your brain is the most receptive.

I have another tip that helps me a lot (I use it as a complement). I put one hand lightly on my whole face and the other on the back of the skull. Using my body awareness I try to be aware of the space under the palate (which is basically the anterior part of the skull's base), and if there is still air, I just try to raise the tongue and fill it -still being aware of the hyoid bone. And then just try to feel the whole skull and tongue as one unit. Maybe it helps you too.

Another one you probably already know,  is to be persistent, consistent and patient, because I believe it's a life journey.

As for the not breathing properly, you can do the alternating nostril breathing right after getting the tongue position, to help you determine at what point you can start breathing. It probably will sound like a slight snore. (I have already "catched" some people doing a similar noise while breathing through the nose, so I wouldn't be so self-conscious about it)

Ok, great, thanks a lot again for all the beautiful advice!