Before you attempt to draw adult faces, it helps to know how to plan a place for everything, sort of like a blueprint. Even though the heads and faces of adults come in many shapes and sizes, the same basic guidelines for proportions apply to almost everyone.
When drawing adult faces, you have to consider the placement of different facial features relative to the overall face shape. This tutorial explains how to examine adult heads and faces, draw the outline of the head, add horizontal lines to the outline, and draw ears and facial features.