For me, if it's worth thinking about it, it's worth writing it down. Doesn't matter if it's a todo list I just came up with, a system diagram, whatever I am currently working on, or thoughts on a human interaction I just witnessed. The act of writing it down guides me in my thinking.

Frankly, at the beginning? Anything you feel like. You can start, perhaps, with Just a title of what you're doing, pomodoros style.

Maybe a note of something you thought but couldn't follow up on that moment.

Diagrams are good. Much easier to think and much better and faster doing by hand. I always get distracted by the tool when I'm drawing in a computer. Even artist-modd

I also make bullet points of general ideas that I'm trying to accomplish.

Doodles.

Important thing is, don't fret. Over time you'll find how it works for you.

Every time you look up something on StackOverflow, refer to the API docs, or refer back to the ticket, use case, or requirements document, make a note of your question and the answer. Even when you stop typing to take a break for a moment, or after pushing code while you wait for the ci/cd pipeline, note down where you are and your last action or change.

Every time you start to write a TODO comment, make a note instead, or also.

Consider Kent’s Beck’s recommendation to write down every decision you make.