As I've said in other threads https://news.ycombinator.com/item?id=38847750#38862450, I highly recommend writing an ELF by hand at least once. It's a great exercise to understand the basic parts of an executable. It's also helpful if you want to go the opposite direction of this article - bottom-up instead of top-down.
Lots of other great discussion in various threads on that other HN post.
Lots of other great discussion in various threads on that other HN post.