So, what do you need to know?
If I would like to draw like this? Everything. Straight from the beginning. But seriously. With drawing skills like this, you could be a video game character and other illustrator. One of my favorite strategy games is Heroes of Might and Magic II from 1996. That game has a hand drawn graphics.
RE: Several stages of drawing a chicken's head with a pencil. Несколько этапов рисования головы курицы карандашом