How do you create realistic creatures?

Also, advice for young illustrators navigating rough seas.

How do you go about creating your paintings of creatures that no longer exist? What steps must you take for the creatures to look as real as they do?

My method is based on the nineteenth century academic approach: thumbnail sketches in black and white and color, studies or photos from costumed models, plein air sketches, maquettes (sculpted miniatures) …