1. Make sure you get good reference for your subject. Try to get photos which actually look like them, Different angles are good.
2. With a front view, try to determine the overall shape of the head. Is it more round than square? Is it long? Is it wide? Does it resemble the shape of an object? (an egg, a pear, a banana, a bottle?)
3. Take note of t he dominate features and make them even more dominant. Make a big nose bigger, wide set eyes wider. I'll sometimes squint my eyes to blur the image making it easier to see the dominant features.
Another trick is to hold the photo up to a round, shiny surface such as an electric kettle. The reflection distorts the features of the face nicely, enabling you to see what they'll look like when exaggerated.
4. Make noticeably small features even smaller. Minimize or Omit
5. Look at how features are arranged on the face. If they're concentrated in the center squeeze 'em even closer together. Are they spread out horizontally? Spread them wider. Which way do the eyebrows slant?
6. Observe your victim's hair line. See where it starts in proportion to the rest of the face. Look at the shape of the hair. A person's hairstyle may be their distinctive trade mark (Donald trump). Look at texture- Is it curly, wavy, thinning, straight, wispy, fuzzy, kinky, non-existent?
7. Study the shapes of the facial features, especially the eyes
8. Try to draw the caricature with as few lines as possibly. This is called economy of line.
9. Make a number of sketches. Fool around with size and alignment of features. Play with shapes. Experiment to see what gives you the best likeness. Put your sketches away. Come back to them later and see if any one 'jump' out at you.
10. Some final tips: try turning your reference photo upside down to give you a fresh look. Something might become a little more obvious to you. To keep the face symmetrical turn your sketch over and look at it from the other side using a light-box. If your subject's face is way out if shape it will immediately become obvious to you.