Ask HN: How do I study optimization?

I went to grad school for computer science and applied math, but optimization was not my focus field. Nowadays, I want to learn both theory and practice for real world optimization problems.

How do I start studying optimization? I understand there are many subtopics within optimizations. I also understand that there are also powerful commercial software with closed source and proprietary (heuristic) algorithms, and these are tougher to study from.

My goal is to understand optimization theory and application well enough to use for real world problems and even write my own optimization software.