These are great!

It's pure mileage in my experience. You just start getting a hang for how certain initial emojis put the eyes too far apart, the mouth down too low, etc.. The way the emoji distorts the construction is based on your particular inclinations as an artist for facial feature placement, so for one artist it will mess up eye placement while for the other it messes up the size of the forehead or something like that. This exercise lives in balance with studies of more traditional head construction and proportion. The traditional stuff shows you where features should sit and how far apart they should be, and the emotive sketches add interest and flexibility to the generally rigid traditional studies.