Record the context briefly
Write the place, equipment and the task agreed with your coach. If surfing in the ocean, note conditions in ordinary language and avoid claiming measurements you did not make. The purpose is to remember the setting, not produce an official forecast record.
Separate observation from interpretation
I missed several take-offs is an observation; I need a smaller board is an interpretation. Keep them separate so you can discuss the second part with an instructor. A useful note preserves the question rather than pretending every session provides its own diagnosis.
Keep one thing that worked
Write down a cue, decision or preparation step you want to repeat. It may be simple and it may not appear on video. Noticing a useful habit gives the next session a practical starting point.
Finish with a next question
Choose a question to ask or a task to repeat. Leave room for the instructor to adjust it to the conditions. A short record that guides your next decision is worth more than a detailed spreadsheet you never open again.