Of all the things in the world, I consider portraits, to be the hardest to draw. There is one reason they are so hard, because, if you make the tiniest mistake it will change the whole portrait and it will not look right. People know exactly what their faces look like, so it is very important to get every feature on the face right.
The proportions of the face are probably the most important part to get right. You could have amazing eyes, an amazing nose, and an amazing mouth was drawn perfectly. But if you misplace them, your portrait will look like an alien. This is a simple proportion diagram, but of course, everyone's face proportions are different. The most common mistakes for face proportions is: eyes are too large, the nose is too small, and the eyebrows are too high.
And lastly, we get to the face features. I drew a very rough example of the face features. Do not forget, lips are slightly wider then nose and every face have different features, happy drawing!