Pattern Recognition / Observation Practice. Try it yourself. Test your IQ!
Get a pen and a piece of paper. Get a reference. Do not use guidelines or make a preliminary sketch. Try to measure the lines using estimation INSIDE YOUR MIND.
Use your brain and eyes only. For example: “This line looks about one inch long in this section. The jaw line is approximately 110 degrees angle…”
Then, try to recognize and mimic the line patterns as accurately as possible to copy the design.