I did use Georgiann’s suggestion of “Brady Bunch squares” by placing a grid over both the photo and my canvas when drawing my initial sketch.
In the video above, the artist makes an observation that really helped me. She used to use tons of layers so she could go back and undo her mistakes, but then she realized that it made a lot more sense to think like an oil painter. “Just paint over it, duh!”
Here’s the source photo:
And just for fun, here’s how it looked about halfway through.