This one is more like an addition to my third tutorial in series. I thought leaving this out intentionally, since there are plenty of tutorials regarding this which can explain things in more detail.
Now there are really many ways to construct the head. One possible way is to use an eye as an unit and locate the facial features according to it. In this case, semi-realistic head is ~4 eyes wide and ~13 eyes high. The head I usually draw apparently is also ~4 eyes wide, but only ~8 eyes due to bigger eyes. I guess it's good for getting started, but I prefer trusting my judgement about right and wrong.
The blue lines indicate medians. Red lines are in my opinion the most important guidelines along the medians. I rarely draw other guidelines than medians, but after sketching the basic shape of the head, I usually draw the ear(s) before other facial features. Note it's position just next to the intersection of two medians. Then I usually follow this simple rule: the ears are on the eye level. The nose is on the same level with earlobe. The space between eyes is one eye wide. I often get satisfying results with only these.
Note that these are only examples. You can play a lot with the facial proportions, if you can make it look justified. You will learn that only trough trial and error.
Then drawing the head from any other angle gets a bit more complicated. On way is to use the two projections as guides to construct the head, but it takes quite a lot of effort. This is why it's so important to be able to see the proportions in three dimensional space, as I mentioned in my first tutorial. Once you can see them, you don't really need guidelines much at all. For me, a sphere for basic head shape and medians are enough.
I truly recommend sketching heads from many different angles every now and then. I have been drawing these heads many times already, which is why I can draw them fairly quickly. You could call it muscle memory.