This picture shows the nasal cavity http://www.zompist.com/kitcons.gif
And here is a description. The description could be much better, but that's academic style. But picture and description indicate that the air passage through the mouth is closed. The air goes through the nasal cavity and the nose. We influence this by drawing in/back the tongue as I've just found out. http://www.pronuncian.com/Lessons/Default.aspx?Lesson=19 If you click "play" you hear the sound.
And the text also says that the vocal cords are vibrating, an important feature. The vocal cords are vibrating when you say a long vowel, e.g. aaaa.
Added: I had a closer look at the ng-problem, that is how we raise the back of the tongue. The description of pronunciation.com says "the back of the tongue lifts", but how? I doubt that anybody can lift the back of the tongue, the formulation is wrong. But you can bury the tip of your tongue as far down below the lower teeth as possible. This raises the back of the tongue automatically, if you want to or not. If you do this and try to say /n/ you produce the ng-sound.