When I was in San Francisco last week, I visited the Eyvind Earle special exhibit at the Walt Disney Family Museum. Eyvind was a background painter and concept artist on Sleeping Beauty and other 1950s Disney features. I love this series of small paintings he made to train his assistants:
Today, something like this would be done digitally in layers. In 1959, he had to paint each layer multiple times to fully demonstrate the technique. No shortcuts.