He's using mannequins in the main picture at the top of the article, so I assume one technique would be to use mannequins as placeholders until you needed to do

He's using mannequins in the main picture at the top of the article, so I assume one technique would be to use mannequins as placeholders until you needed to do the distinctly human parts of the painting (hands, faces), then you could substitute a person into place and paint that part.