A pocket notebook
A useful notebook does not need an elaborate system. Start with a date, a place, and something you noticed. The point is to leave a small trace while the detail is still fresh.
Write what you can see before writing what it means. “Three pale leaves beside the gate” is a better beginning than trying to sum up the whole season.
Leave room for unfinished thoughts. A page can hold a sentence, a rough sketch, or a question. Returning to it a week later often reveals connections that were not obvious at first.