Good notes are the difference between a professional who remembers and one who asks the same questions every visit. Write them at the end of the appointment while the detail is intact — five minutes later, not five days.
The things worth recording every time:
- The formula, the developer, the timing and the result, including what you would change
- Products used and how the hair, skin or nails responded
- Sensitivities, reactions and patch-test dates
- How long it actually took, so the booking duration can be corrected
- What they said they wanted next time, and their usual interval
The rule for everything else: write it as though the client will read it, because one day they might — clients can ask for the information a business holds about them, and in many places you have to provide it.
So keep opinions about the person out of it. 'Client is difficult' helps nobody and reads appallingly; 'wants to be told before any extra cost' is the same observation made useful. Health information beyond what the service requires does not belong there at all, and neither does anything they told you in confidence about their life.
Notes about no-shows and late cancellations are fine and worth keeping — they are facts about the booking, and they are what tell you when somebody should be on a deposit.