I used to teach this shit. And my opinion is that people make this more complicated than it is.

Want to learn? Awesome!

So… uh real shit, try to do quick drawings / paintings (using a timer) with a model (can be living or not). Think 1 minute, 30 seconds, 10 seconds. Doing this consistently (like drawing people in the public transport, which is time limited by default) will make you better at observing and putting those observations to paper.

The other key is to TRY to draw the model and not your IDEA of the model. This often implies painting/ drawing a bunch of amorphous blotches as opposed to straight clearly demarcated physiological features.

Happy arting!