There is a particular kind of useful recipe note that would never make it into a cookbook: use the bigger pan. Or: more lemon next time.
A kitchen notebook can hold those small discoveries without needing to become a project of its own.
Write down what changed
Start with the name of the dish and the date. Add one or two observations while you still remember them. Did the carrots take longer than expected? Was the sauce better with a spoonful of pasta water?
Useful notes are specific. “Good” is pleasant to read later, but “roast for five minutes longer” helps you cook.
Leave room for accidents
A substitution that worked deserves a line too. So does the combination you would rather not repeat.
You can keep this on your phone, on index cards, or in a notebook with flour in the spine. The format matters less than being able to find the note when you next stand in front of the stove.