I write down:

- To-do items (with empty checkboxes)

- Notes about what I did, every so often. Or what I talked to someone about, what was decided.

- If I'm programming, I try to have a kind of plan for the next fifteen minutes / hour in a few sentences. "Going to refactor this now." "Updating the state here so it can hold this information." "Adding a component for this". Just so that I do think about what I'm going to do for a bit.

That sort of thing.

Apart from the to-do's the main point is to keep my focus, when I'm writing thoughts on paper I'm not on Hacker News. It doesn't matter all that much what the writing is, to me.