A useful kitchen note is often one sentence: cut the potatoes smaller, use less mustard, or save more pasta water. It tells you what to do next time without turning dinner into paperwork.
Write the ingredient brand when it made a noticeable difference. Plant yogurt, tofu, and canned tomatoes can vary enough that the name is more informative than a vague comment about the recipe being better.
Separate preference from a technical problem. “Too lemony for us” calls for a different adjustment from “the sauce split when it boiled.” Note the amount or step involved while you still remember it.
You do not need to record every meal. Save the observations that remove a decision later. A short page of things you have learned about your own pans and ingredients is more useful than a perfectly organized notebook you rarely open.