A good way to practice is to get a reference picture and write down what you notice about the person. Are they tall, lanky, how do they style their hair, what colour is it, is their face rounded or gaunt, the shape of their eyes, emotional impressions (e.g. do they have a wary gaze, stern jaw, laugh lines), describe their facial hair, if they have thin or bushy eyebrows, if their nose was broken, any piercings or facial markings, how pronounced their cheeks, jaw, and chin are, and so on.