Face it: drawing faces for fashion illustration is important but not easy.
Granted, figure structure (anatomy, movement, pose) is the most important aspect of fashion illustration. The entire apparel design aspect of fashion design is about three-dimensional shape of a human body. This is why some fashion illustration styles leave out face completely, replacing it with an abstract sign or even a blank space.
However, drawing a face is important in fashion illustration and requires special attention.
While learning or advancing your fashion figure drawing skills, keep in mind that there are two main aspects which make drawing faces in fashion especially important:
Face could be illustrated from scratch. Drawing face step by step requires some knowledge of figure drawing, experience and practice. But most importantly, a fashion illustrator needs to know why face has to be a part of the fashion illustration in the first place.

Is face merely a stylistic touch or is it a focal point of the illustration? A face drawing could be rendered with some hints of dimensionality or a face could be illustrated in a completely flat manner with minimum or no rendering at all, depending on the style of illustration.
Face drawing could be extremely stylized when only a brushstroke hinting lips, an eye or brow will do the trick.
Face could also be completely replaced by a stylized symbol, a little something in fashion illustration which will mark or substitute face in a fashion sketch.
Face drawing could be created using templates or a fashion croquis (fashion croqui). (see example of fashion croquis for fashion figure drawing)
It does not mean that croquis has to be traced blindly: instead, it could be creatively interpreted by a fashion illustrator. Drawing faces with fashion croquis requires fundamentally two main things:
The most important skill in using a fashion croquis (fashion croqui) is to leave out some parts of it. Do not reproduce entire face, but only lips or only an eye, or only the shape of the head.