Answer:
Explanation:
He used the grid to section out the picture. He would put a grid on top of the picture he was copying and had a to-scale grid on top of the canvas he would paint on. Then, in the grids on the canvas he would paint each section as he saw in the photo.
If you watch "Chuck, Up Close" by World Science Festival they kinda answer your question :)
*also your pdf doesn't work :(
