@I AM RETARDED!
I quickly sketched the image and highlighted in red important spots to focus on. This is not universal. They usually vary depending on the source. Finding what’s unique about the face you want to draw is crucial. And finding what’s not is important too. For example when I look at this girl particularly, I notice the dominant arch of her brow, the curving of her full lips, the shape of her eyelids and the shape of her hairline. But the nose is quite unremarkable and can be omitted without loosing the likeness.
When you want to capture someone’s likeness, you really just have to take a good look at their dominant features.
Side note - the position of their pupils is important.
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