Frank Netter, the dean of medical illustration, offers universally useful advice in this archival video:
"There's nothing quite as frightening as a blank piece of paper in front of you, and you know you've got to put a picture on it. And you can sit there and look at it all day long and puzzle 'How should I begin? Where should I begin? What shall I do about this? And you'll never get a picture done. You must begin! You must make some strokes. You may throw it away later. But you must begin to paint!"
Frank Netter's book: The Netter Collection of Medical Illustrations: The Endocrine System: Volume 2, 2e (Netter Green Book Collection)