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.