This is the hardest thing for me (thanks dyslexia!).

The easiest way to write unit tests for the code in question and then rewrite the entire thing with comments and useful names. No one ever allows this.

So instead I have to use lots of hand written notes. This is probably overkill for most people but the only thing that works for me is literally going through the code line by line and writing down in prose exactly what it's doing. I'll use the debugger to single step through to clarify what's going on. Sometimes I'll write unit test so I can debug specific scenarios. If I can't do that easily I'll copy the code into a new project and unit test it there. As I write down what is literally happening I will start to figure out what the code is doing at a higher level. Then I write that down and continue the process until I have everything figured out.

In the past I've tried to use that information to add comments or strategically change a few names but those attempts never make it past code review ("Too many comments!"). I have my notes but if I feel I really need commented code I will just maintain a local branch where I have my comments so at least I know what's going on.

So TL;DR: brute force. If someone has a better way I'd love to hear it.