Now this guy is as boring as all get out but if you can get past that the video is a good overview of what you should be trying to achieve when you are doing back stroke.
Since he is no beginner and in fact an all American Swimmer and therefore one can say that his stroke is probably right on for him; but I have to say that I find that he puts his head far too deep in the water for what I would consider optimal for most beginners.
I would much prefer that a beginner only allows the top of their head to skim just under the water.The reason for this is beginners tend to keep their heads either too deep or too high out of the water.
If your head is too deep under the water then the average beginner is going to have trouble getting their head high enough in time to take a clear breath of air and may breath in some water.
If your head is too shallow in the water the glide part of the stroke become difficult.
In both cases the beginner may become discouraged and that is not good for practice.
So it is best to let your head skim just under the water.