Use familiar fields
Write the date, confirmed place, duration if known and the preparation detail you want to remember. Keep the format consistent. Approximate information should be marked as approximate. A blank space is better than a precise number reconstructed from a general impression.
Separate the explanation
An observation and its possible cause belong in different lines. A single session rarely establishes a pattern. Keep a question for later reading rather than turning your first idea into a conclusion. This also makes it easier to revisit the entry honestly.
Read it again
At the end of the month, choose one detail to keep and one to change. The fieldbook need not document everything. It should help you prepare more clearly and remember the actual experience, including ordinary outings without a remarkable result.